This tutorial assumes you are working on a repository TheRepo whose main instance is hosted by an organization TheOrg with a main branch TheBranch. (TheRepo, TheOrg, and TheBranch are placeholder values that you should fill in based on the repo you're using.)
1. Fork TheRepo on GitHub to create your fork TheFork (only needs to be done once):
In the top-right corner of the page, click Fork.
See example at GitHub: Fork A Repo
(Don't actually follow the exact steps on this page; use it as an example and apply the steps to the repo you're using.)
2. Clone a local version of the code from TheFork (and follow any other necessary installation instructions for the repo):