See Moodle for the due date.
There is a file in the course repository assignments/guestbook.md. Fork the course repository on GitHub, create a branch, and add some kind of content to this file. See the instructions at the top of that document for inspiration.
Once you’ve done this, commit your changes (remember to write a good commit message) and open a “Pull Request” (PR) on the main GitHub repository. In the PR description, include a one-sentence description of your change.
Submit the link to your PR on Moodle.