Clone your forked repository to your local machine
"Cloning" means downloading the repository contents to your local machine, where you can edit them.
If you are not familiar with Git then an easy starting point is to use a Git client with a graphical user interface. A free and easy to use option is GitHub Desktop. It comes with a user guide on how to install it and how to authenticate with your GitHub Account.
You can now proceed to follow the GitHub Documentation on cloning your forked repository, or the GitHub Desktop specific guide with screenshots.
Once you have the repository downloaded to a local directory on your machine you can proceed to the next step.