diff --git a/.gitignore b/.gitignore
index 8021355..b21e050 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,8 +2,9 @@
 *.retry
 
 # ---> Generated slides (built by marp on server or locally via build-slides.sh)
-slides/*.html
-slides/*.pdf
+#      Deck *sources* also live under slides/, so ignore only the build output.
+slides/**/*.html
+slides/**/*.pdf
 
 # ---> MkDocs build output
 site/
diff --git a/CLAUDE.md b/CLAUDE.md
index 80bd579..6f6addd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -27,13 +27,15 @@ docs/             # everything here is built and shipped to docs.makerfloss.eu
   hardware/       # auto-indexed per-host frontmatter (srv01..srv05, makerfloss.eu)
   services/       # auto-indexed per-service frontmatter (docs, forgejo, …)
   infrastructure/ # labdesign, VPS/DNS, etc.
-  presentations/  # Marp decks (build-slides.sh)
+  presentations/  # Marp decks, flat URLs (build-slides.sh)
 notes/            # repo-only working material, not built
   meetings/       # meeting notes (Danish allowed)
   todo/           # task lists, working norms, wishlist, services
   dev/            # internal plans/ and specs/
   communications/ # community comms artifacts (Facebook posts, etc.)
 sandbox/          # scratch / pipeline fixtures (e.g. test-mermaid.md)
+slides/           # build output — AND Marp deck sources whose URL is a path,
+                  # e.g. slides/tappaas/how-to/new-module/index.md
 ```
 
 ## Infrastructure
diff --git a/build-slides.sh b/build-slides.sh
index e5c74da..34d3d6a 100755
--- a/build-slides.sh
+++ b/build-slides.sh
@@ -1,4 +1,14 @@
 #!/usr/bin/env bash
+# Build every Marp deck in the repository into static HTML.
+#
+# A deck is any *.md carrying `marp: true` in its front matter. Two source
+# roots, two output conventions:
+#
+#   docs/presentations/<name>.md   ->  <name>.html          (flat, historical)
+#   slides/<path>/<name>.md        ->  <path>/<name>.html   (path mirrors URL)
+#
+# So slides/tappaas/how-to/new-module/index.md is served at
+# https://slides.makerfloss.eu/tappaas/how-to/new-module/ .
 set -euo pipefail
 
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
@@ -8,53 +18,74 @@ REPO_WEB_URL="${REPO_WEB_URL:-https://forgejo.makerfloss.eu/sjat/MakerFLOSS}"
 
 mkdir -p "$OUTPUT_DIR"
 
-# Wipe previously built slides so deleted/de-tagged .md files disappear.
-find "$OUTPUT_DIR" -maxdepth 1 -name "*.html" -delete
-
-# Find all markdown files with marp: true frontmatter
-SLIDES=()
+# Collect decks as "<repo-relative source>|<output-relative target>" pairs.
+DECKS=()
 while IFS= read -r f; do
-  SLIDES+=("$f")
-done < <(grep -rl "^marp: true" "$REPO_ROOT/docs" --include="*.md" 2>/dev/null || true)
+  rel="${f#"$REPO_ROOT"/}"
+  DECKS+=("$rel|$(basename "${rel%.md}").html")
+done < <(grep -rl "^marp: true" "$REPO_ROOT/docs" --include="*.md" 2>/dev/null | sort || true)
 
-if [ ${#SLIDES[@]} -eq 0 ]; then
-  echo "No marp presentations found in docs/."
+if [ -d "$REPO_ROOT/slides" ]; then
+  while IFS= read -r f; do
+    rel="${f#"$REPO_ROOT"/}"
+    out="${rel#slides/}"
+    DECKS+=("$rel|${out%.md}.html")
+  done < <(grep -rl "^marp: true" "$REPO_ROOT/slides" --include="*.md" 2>/dev/null | sort || true)
+fi
+
+if [ ${#DECKS[@]} -eq 0 ]; then
+  echo "No marp presentations found."
   exit 0
 fi
 
-echo "Found ${#SLIDES[@]} presentation(s):"
-printf '  %s\n' "${SLIDES[@]}"
+echo "Found ${#DECKS[@]} presentation(s):"
+printf '  %s\n' "${DECKS[@]%%|*}"
 
-# Create temp output directory for Docker fallback
-TEMP_OUTPUT=$(mktemp -d)
-chmod 777 "$TEMP_OUTPUT"
-trap "rm -rf '$TEMP_OUTPUT'" EXIT
+# Wipe previously built HTML so deleted/de-tagged decks disappear. Only *.html
+# is removed, so deck sources living inside slides/ are never touched.
+find "$OUTPUT_DIR" -name "*.html" -type f -delete
 
-if command -v marp &>/dev/null; then
-  for slide in "${SLIDES[@]}"; do
-    BASENAME=$(basename "${slide%.*}")
-    marp --html --output "$OUTPUT_DIR/${BASENAME}.html" "$slide"
-  done
-else
-  echo "marp not found locally — using Docker (marpteam/marp-cli)..."
-  for slide in "${SLIDES[@]}"; do
-    REL_SLIDE="${slide#${REPO_ROOT}/}"
-    BASENAME=$(basename "${slide%.*}")
+render() {                          # render <source-rel> <target-rel>
+  local src="$1" dst="$2"
+  mkdir -p "$OUTPUT_DIR/$(dirname "$dst")"
+  if command -v marp &>/dev/null; then
+    marp --html --output "$OUTPUT_DIR/$dst" "$REPO_ROOT/$src"
+  elif command -v npx &>/dev/null; then
+    npx --yes @marp-team/marp-cli --html --output "$OUTPUT_DIR/$dst" "$REPO_ROOT/$src"
+  else
+    # marpteam/marp-cli drops privileges to its own "marp" user, so the mounted
+    # output tree has to be writable by anyone — and --user must NOT be passed.
+    chmod -R a+rwX "$OUTPUT_DIR"
     docker run --rm \
       -v "$REPO_ROOT":/home/marp/app:ro \
-      -v "$TEMP_OUTPUT":/home/marp/output \
-      marpteam/marp-cli --html --output "/home/marp/output/${BASENAME}.html" "$REL_SLIDE"
-  done
-  cp "$TEMP_OUTPUT"/*.html "$OUTPUT_DIR/" 2>/dev/null || true
+      -v "$OUTPUT_DIR":/home/marp/out \
+      marpteam/marp-cli --html --output "/home/marp/out/$dst" "$src"
+  fi
+}
+
+if ! command -v marp &>/dev/null && ! command -v npx &>/dev/null; then
+  if command -v docker &>/dev/null; then
+    echo "marp not found locally — using Docker (marpteam/marp-cli)..."
+  else
+    echo "error: need one of marp, npx or docker on PATH" >&2
+    exit 1
+  fi
 fi
 
-# Inject mermaid.js into any HTML that contains mermaid code blocks.
+for deck in "${DECKS[@]}"; do
+  render "${deck%%|*}" "${deck#*|}"
+done
+
+# Post-process each built page: highlight unfinished `TODO ...` markers, and
+# turn mermaid code blocks into live diagrams.
+#
 # Marp emits fenced mermaid blocks as <pre><code class="language-mermaid">.
-# The script finds those elements, replaces them with <div class="mermaid">,
-# then loads and runs mermaid.js from CDN.
-inject_mermaid() {
+# The script replaces those elements with <div class="mermaid">, then loads and
+# runs mermaid.js from CDN.
+postprocess() {
   local html_file="$1"
   python3 - "$html_file" << 'PYEOF'
+import re
 import sys
 
 path = sys.argv[1]
@@ -73,25 +104,33 @@ await mermaid.run();
 
 with open(path, encoding='utf-8') as f:
     content = f.read()
-new_content = content.replace('</body>', snippet + '\n</body>', 1)
-if new_content == content:
-    print(f"Warning: </body> not found in {path}", file=sys.stderr)
-    sys.exit(1)
+
+# Inline <code>TODO…</code> -> <code class="todo">…</code>; decks style that class.
+content, todos = re.subn(r'<code>(TODO\b[^<]*)</code>', r'<code class="todo">\1</code>', content)
+
+if 'class="language-mermaid"' in content:
+    if '</body>' not in content:
+        print(f"Warning: </body> not found in {path}", file=sys.stderr)
+        sys.exit(1)
+    content = content.replace('</body>', snippet + '\n</body>', 1)
+    print(f"  Injected mermaid.js into {path}")
+
 with open(path, 'w', encoding='utf-8') as f:
-    f.write(new_content)
+    f.write(content)
+
+if todos:
+    print(f"  {todos} unfinished TODO marker(s) in {path}")
 PYEOF
 }
 
 if command -v python3 &>/dev/null; then
-  for html_file in "$OUTPUT_DIR"/*.html; do
-    [ -f "$html_file" ] || continue
-    if grep -q 'class="language-mermaid"' "$html_file"; then
-      inject_mermaid "$html_file"
-      echo "  Injected mermaid.js into $(basename "$html_file")"
-    fi
-  done
+  # The landing index.html is generated further down, so every *.html present
+  # right now is a built deck.
+  while IFS= read -r html_file; do
+    postprocess "$html_file"
+  done < <(find "$OUTPUT_DIR" -name "*.html" -type f)
 else
-  echo "Warning: python3 not found — skipping mermaid injection"
+  echo "Warning: python3 not found — skipping post-processing"
 fi
 
 # Regenerate index.html listing every built deck.
@@ -113,9 +152,10 @@ INDEX="$OUTPUT_DIR/index.html"
   printf '<p class="sub">Built from <a href="%s">%s</a></p>' \
     "$REPO_WEB_URL" "${REPO_WEB_URL#https://}"
   printf '<ul>'
-  find "$OUTPUT_DIR" -maxdepth 1 -name "*.html" ! -name "index.html" | sort | while IFS= read -r html; do
-    name=$(basename "$html" .html)
-    printf '<li><a href="%s">%s</a></li>' "$(basename "$html")" "$name"
+  for deck in "${DECKS[@]}"; do
+    href="${deck#*|}"
+    label="${href%/index.html}"; label="${label%.html}"
+    printf '<li><a href="%s">%s</a></li>' "$href" "$label"
   done
   printf '</ul>'
   printf '<footer>Last built: %s &middot; <a href="https://marp.app">Marp</a></footer>' "$(date -Iseconds)"
diff --git a/docs/services/slides.md b/docs/services/slides.md
index ea00c5b..5278ce3 100644
--- a/docs/services/slides.md
+++ b/docs/services/slides.md
@@ -11,4 +11,18 @@ tls: letsencrypt
 
 ## Notes
 
-Slide-deck site. Decks are authored as Marp markdown in `docs/presentations/` and compiled to HTML by `build-slides.sh` (CI invokes it via the [marp](marp.md) toolchain). Built output is rsynced to the VPS and served alongside [docs](docs.md).
+Slide-deck site. Decks are authored as Marp markdown and compiled to HTML by
+`build-slides.sh` (CI invokes it via the [marp](marp.md) toolchain). Built output is rsynced to
+the VPS and served alongside [docs](docs.md).
+
+Two source roots, two output conventions:
+
+| Source | Published at |
+| --- | --- |
+| `docs/presentations/<name>.md` | `https://slides.makerfloss.eu/<name>.html` |
+| `slides/<path>/<name>.md` | `https://slides.makerfloss.eu/<path>/<name>.html` |
+
+Use `slides/` when a deck belongs to a named series and the URL should read like a path — for
+example `slides/tappaas/how-to/new-module/index.md` is served at
+`https://slides.makerfloss.eu/tappaas/how-to/new-module/`. Only the markdown is committed; the
+generated HTML under `slides/` is git-ignored.
diff --git a/slides/tappaas/how-to/new-module/index.md b/slides/tappaas/how-to/new-module/index.md
new file mode 100644
index 0000000..57779c9
--- /dev/null
+++ b/slides/tappaas/how-to/new-module/index.md
@@ -0,0 +1,232 @@
+---
+marp: true
+theme: gaia
+class: invert
+paginate: true
+title: How to implement a TAPPaaS module
+description: Walk-through of adding a new module to the TAPPaaS repository
+---
+
+<style>
+section { font-size: 26px; }
+h1 { font-size: 1.5em; }
+h2 { font-size: 1.15em; }
+pre { font-size: 0.72em; line-height: 1.35; }
+table { font-size: 0.8em; }
+th, td { padding: 0.25em 0.6em; }
+/* Unfinished content, loud on purpose: an unfinished deck should never be
+   presented by accident. Written in the markdown as `TODO: ...`. */
+code.todo { color: #ff8a80; font-weight: 600; }
+div.mermaid { display: flex; justify-content: center; }
+</style>
+
+<!-- _paginate: false -->
+
+# How to implement a TAPPaaS module
+
+From empty directory to merged pull request
+
+<!--
+Speaker notes go in HTML comments; press `p` in the browser for presenter view.
+TODO: 30-second framing — who is in the room, what they walk out able to do.
+-->
+
+---
+
+## Who this is for
+
+- You want to add an **app** or a **foundation** capability to TAPPaaS
+- You can read shell, JSON, and a little Nix
+- You have a TAPPaaS test environment you are allowed to break
+
+**Prerequisites**
+
+- A clone of `TAPPaaS/TAPPaaS`, branch `TODO: ADR007 or stable`
+- SSH access to a test site
+- `TODO: tooling list`
+
+---
+
+## Vocabulary in 60 seconds
+
+| Term | Means |
+| --- | --- |
+| **Module** | One installable unit under `src/apps/` or `src/foundation/` |
+| **Service** | A part of a module with its own lifecycle scripts |
+| **Manager** | Owns desired state for a domain |
+| **Controller** | Applies desired state to one concrete technology |
+
+`TODO: tighten against ADR-007 taxonomy; drop the terms that do not matter here`
+
+---
+
+## Anatomy of a module
+
+```text
+src/apps/<module>/
+├── README.md          # what it is, why it exists
+├── DESIGN.md          # how it is put together
+├── INSTALL.md         # operator-facing install notes
+├── UPGRADE.md         # operator-facing upgrade notes
+├── AUTHORS.md         # who to ask
+├── LICENSE
+├── <module>.json      # declaration: the module's contract
+├── <module>.nix       # runtime definition
+├── install.sh         # create it
+├── update.sh          # move it forward
+├── test.sh            # prove it works
+└── services/<svc>/    # per-service install/update/test/delete
+```
+
+---
+
+## The path from nothing to merged
+
+```mermaid
+flowchart LR
+  A[Scaffold from<br/>00-Template] --> B[Declare<br/>module.json]
+  B --> C[Define runtime<br/>module.nix]
+  C --> D[install.sh]
+  D --> E[services/]
+  E --> F[update.sh]
+  F --> G[test.sh]
+  G --> H[Docs]
+  H --> I[PR + CI]
+```
+
+---
+
+## Step 1 — Scaffold
+
+```bash
+cp -r src/apps/00-Template src/apps/<module>
+```
+
+- Rename `template.json` → `<module>.json`, `template.nix` → `<module>.nix`
+- Read `README-template.md`, then delete it
+- `TODO: is there a scaffold script? if not, should there be?`
+
+---
+
+## Step 2 — Declare the module
+
+`<module>.json`
+
+```json
+{
+  "TODO": "smallest working declaration"
+}
+```
+
+- What the module *is*, not how it is built
+- `TODO: required keys, optional keys, validation`
+
+---
+
+## Step 3 — Define the runtime
+
+`<module>.nix`
+
+```nix
+# TODO: minimal example
+```
+
+- `TODO: what belongs in nix vs. what belongs in install.sh`
+
+---
+
+## Step 4 — `install.sh`
+
+- Idempotent: safe to run twice
+- Fails loudly, never half-way
+- `pre-install.sh` when something must exist first
+
+```bash
+# TODO: skeleton with the house conventions
+```
+
+---
+
+## Step 5 — Services
+
+```text
+services/<svc>/install-service.sh
+services/<svc>/update-service.sh
+services/<svc>/test-service.sh
+services/<svc>/delete-service.sh
+```
+
+- One service = one thing with its own lifecycle
+- `TODO: when to split into services vs. keep it in the module`
+
+---
+
+## Step 6 — `update.sh`
+
+- Upgrade in place, no data loss
+- Version pinning: `TODO`
+- Rollback story: `TODO`
+
+---
+
+## Step 7 — `test.sh`
+
+- Runs against a real environment, not a mock
+- Exit non-zero on failure, quiet on success
+- `TODO: what CI runs, and what only a human can check`
+
+---
+
+## Step 8 — Documentation
+
+| File | Answers |
+| --- | --- |
+| `README.md` | What is this and why would I install it? |
+| `DESIGN.md` | How is it built, what were the trade-offs? |
+| `INSTALL.md` | How do I install it, step by step? |
+| `UPGRADE.md` | What breaks between versions? |
+
+Docs are synced to the docs site — a missing README is a missing page.
+
+---
+
+## Try it on a test site
+
+```bash
+# TODO: the real loop
+```
+
+1. Deploy to the test environment
+2. Run `test.sh`
+3. Break it on purpose, run it again
+
+---
+
+## Submit
+
+- Branch: `TODO: naming convention`
+- One module per pull request
+- CI must be green
+- Review checklist:
+  - [ ] Idempotent install
+  - [ ] `test.sh` passes on a clean site
+  - [ ] Docs complete
+  - [ ] `TODO`
+
+---
+
+## Pitfalls
+
+- `TODO: the three mistakes reviewers actually see`
+- `TODO`
+- `TODO`
+
+---
+
+## Where to go next
+
+- `src/apps/00-Template/` — the canonical starting point
+- `docs/ADR/ADR-007 - TAPPaaS Taxonomy.md`
+- `TODO: contribution guide link`
+
+**Questions?**