Click on the "Use this template" button to copy the template to your namespace.
The codelab entry point is the README. You can put your codelab content including assets under the lessons directory. The codelab manifest.json holds the navigation information and more. Link your lessons in that file in order to render the navigation.
The template codelab is based on the Project Helix https://www.project-helix.io/. Follow instructions as explained.
npm i -g @adobe/helix-cli && hlx up is enough to get you started. Make sure to have at least npm@6.14.x installed and an initial commit.
You can visit https://[REPO_NAME]-[REPO_OWNER].project-helix.page to see it running.
Demo: https://adobeio-codelabs-template-adobedocs.project-helix.page/
Make sure to push your changes before sharing the URL.
Once your codelab is ready, submit a PR to add it to the index at https://github.com/AdobeDocs/adobeio-codelabs/blob/gh-pages/actions/index.json