diff --git a/.ci/scripts/build-all-dev-envs.sh b/.ci/scripts/build-all-dev-envs.sh
new file mode 100755
index 0000000..20802dc
--- /dev/null
+++ b/.ci/scripts/build-all-dev-envs.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env sh
+PROJECT_FLAKES_DIR="project-flakes"
+
+# Print lines of this script as they're executed, and exit on any failure.
+set -ex
+
+# Loop through each sub-directory in 'project-flakes'
+for project in "$PROJECT_FLAKES_DIR"/*/ ; do  
+  # Remove the trailing '/'
+  project=${project%*/}
+  # Extract the project name
+  project=${project##*/}
+  # Echo the project name for logging
+  echo "Running nix develop on project: $project"
+  # Extract the git URL of the project. We often need the files of the
+  # project locally in order to build the development shell:
+  url=$(grep "^# ci.project-url:" "$PROJECT_FLAKES_DIR/$project/default.nix" | awk -F': ' '{print $2}')
+  echo "Cloning repo: $url"
+  # Clone the project to a directory with the same name.
+  git clone --depth 1 -q "$url" "$project"
+  # Enter the project directory.
+  cd "$project"
+  # Show the generated outputs of the flake.
+  nix flake show ..
+  # Attempt to build and enter the development environment,
+  # then immediately exit the built shell.
+  nix develop --impure ..#"$project" -c "true"
+  # Leave the project directory.
+  cd ..
+  # Delete the project directory.
+  rm -rf "$project"
+done
\ No newline at end of file
diff --git a/.github/workflows/build-projects.yaml b/.github/workflows/build-projects.yaml
new file mode 100644
index 0000000..8708a63
--- /dev/null
+++ b/.github/workflows/build-projects.yaml
@@ -0,0 +1,13 @@
+---
+name: "Build project flakes"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+      - uses: DeterminateSystems/nix-installer-action@bc7b19257469c8029b46f45ac99ecc11156c8b2d # v6
+      - uses: DeterminateSystems/magic-nix-cache-action@8a218f9e264e9c3803c9a1ee1c30d8e4ab55be63 # v2
+      - run: .ci/scripts/build-all-dev-envs.sh
\ No newline at end of file
diff --git a/README.md b/README.md
index 42a8e2a..a15a967 100644
--- a/README.md
+++ b/README.md
@@ -103,3 +103,38 @@ provided by this flake's outputs:
 Note that `composeShell` takes a list as an argument. You can provide multiple
 project names in this list, and `composeShell` will build a development shell
 dependencies from all projects combined together.
+
+## Development
+
+To add a new project to this repo, create a directory with its name under
+[`project-flakes/`](project-flakes/), and inside of it create a `default.nix`
+file. This file is not a flake itself, but a function written in the nix
+language that returns a devenv module. A basic example of a `default.nix`:
+
+```nix
+# The below line tells CI where to clone the project from when testing
+# the build of your devenv module. Any git URL is supported.
+# ci.project-url: https://github.com/foo/bar
+
+# Function arguments.
+{ pkgs, ... }:
+
+# The returned devenv module attribute set.
+{
+  # Set some devenv options...
+  packages = with pkgs; [ sqlite mdbook ]
+  languages.c.enable = true;
+  # ...
+}
+```
+
+See [devenv's flake guide](https://devenv.sh/guides/using-with-flakes/) for an
+introduction; the attribute sets returned by your function are what get slotted
+into the `modules` attribute under `devenv.lib.mkShell`. [`flake.nix`](flake.nix)
+is where all that happens.
+
+[Devenv's reference](https://devenv.sh/reference/options/) has a list of all
+available options you can specify.
+
+The [Nix Language basics](https://zero-to-nix.com/concepts/nix-language) is a
+recommended read.
\ No newline at end of file
diff --git a/project-flakes/complement/default.nix b/project-flakes/complement/default.nix
index 29d78ac..989f4f2 100644
--- a/project-flakes/complement/default.nix
+++ b/project-flakes/complement/default.nix
@@ -1,3 +1,4 @@
+# ci.project-url: https://github.com/matrix-org/complement
 { pkgs, ... }:
 
 {
diff --git a/project-flakes/element-web/default.nix b/project-flakes/element-web/default.nix
index 61d8ef5..c54040d 100644
--- a/project-flakes/element-web/default.nix
+++ b/project-flakes/element-web/default.nix
@@ -1,3 +1,4 @@
+# ci.project-url: https://github.com/vector-im/element-web
 { pkgs, ... }:
 
 {
diff --git a/project-flakes/synapse/default.nix b/project-flakes/synapse/default.nix
index 5720200..2191ec7 100644
--- a/project-flakes/synapse/default.nix
+++ b/project-flakes/synapse/default.nix
@@ -1,3 +1,4 @@
+# ci.project-url: https://github.com/matrix-org/synapse
 { pkgs, ... }:
 
 {
diff --git a/project-flakes/sytest/default.nix b/project-flakes/sytest/default.nix
index 017ec67..07af257 100644
--- a/project-flakes/sytest/default.nix
+++ b/project-flakes/sytest/default.nix
@@ -1,3 +1,4 @@
+# ci.project-url: https://github.com/matrix-org/sytest
 { pkgs, ... }:
 
 {