This demonstration uses the Unity Test Tools Repo.
There is a sidebar on the left-hand side of the repository page, with an Actions section:
Click the Fork button and fill out the form you are redirected to, then click Fork repository:
When you have finished forking the repo, you’ll be taken to the landing page for your new fork:
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.