1. Clone a local copy of the TiddlyWiki5 GitHub repository from https://github.com/TiddlyWiki/TiddlyWiki5
2. Open a command line terminal and change the current working directory to the root of the TiddlyWiki5 repo
3. Type npm link (Windows) or sudo npm link (Mac/Linux) to tell npm to use this copy of the repo as the globally installed one
4. Inside the root, you can launch TiddlyWiki like this:
   tiddlywiki editions/tw5.com-server --listen

After this procedure you can work with TiddlyWiki5 via npm as though it had been installed in the usual way with npm install -g tiddlywiki.

Update the clone from time to time in order to ensure that you have the latest code.