Start a manual testing guide

guides/manual-testing.md

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+### Manually testing the Scaffolding tool
+
+#### Ensuring you're using the right Scaffolding version
+
+With the Scaffolding repository checked out, switch to the Git revision you want to test.
+
+Start the Nix development environment for the Scaffolding tool using:
+
+```
+nix develop --override-input "versions/scaffolding" .
+```
+
+This will give you a shell which includes the Scaffolding built from your local repository.
+
+Stay in the current Nix shell and change directory to somewhere that you want to put your test project. 
+For example, `cd /tmp`. You're now ready to scaffold a new app with `hc-scaffold web-app`.
+
+When following the instructions that the Scaffolding outputs, ignore `nix develop` and instead run:
+
+```
+nix develop --override-input "versions/scaffolding" <path-to-local-scaffolding-clone>
+```
+
+Which will ensure that you keep using the updated Scaffolding tool inside the scaffolded hApp environment.
+
+Now you can proceed with testing your changes as needed.