You need to know how to create Commits and you need to have created at least one of them.

We will use the commit(s) we have created (e.g. the example in create Commits) and push the changes to GitHub.

Check the current status

Link to check-the-current-status

If you run git status you can see how many commits your local clone is ahead/behind your fork.

In this case we are ahead by 1 commit:
git status call

Now we run git push to push our changes to the remote branch.
You might be asked for your GH username and password.
Careful: When you use TwoFactor Authorization (2FA) you need to create and use a Personal Access Token instead of your GH password!

Credentials dialog

To verify that the commit is actually on the fork, you can check the commit list that should now have the commit in there: Commit List with commit

Now that you have pushed your changes to your fork, you can use GH to create a Pull request.
This is the same procedure as if you were using the online editor to edit files.