You may want to review the github PR submission workflow.
In general
1) fork the repo to your own github projects
2) git clone your project to a local checkout dir
3) make a PR branch in the local checkout dir
4) make your changes locally
5) do a 'git diff' to make sure they make sense
6) check your changes into the PR branch
7) push the PR branch to your own githpub fork of the project
open a pull request to pull from your fork to main of the upstream repo
All of the above requires a good working knowledge of both git and github.