Click on Fork to fork the parent repository to your account.
A GitHub fork is a copy of a repository (repo) that sits in your account rather than the account from which you forked the data from. Once you have forked a repo, you own your forked copy. This means that you can edit the contents of your forked repository without impacting the parent repo.
Make the changes you want to (in your forked repository and not the original one!) and click on commit changes
A commit, or "revision", is an individual change to a file (or set of files). When you make a commit to save your work, Git creates a unique ID (a.k.a. the "SHA" or "hash") that allows you to keep record of the specific changes committed along with who made them and when.
Create a pull request to propose and collaborate on changes to a repository. These changes are proposed in a branch, which ensures that the default branch only contains finished and approved work.
Once you have commited the changes, go to Pull request and click on New Pull request
After clicking, you'll be directed to following screen, Here click on create pull request.
You can refer following link for more information on pull requests. https://docs.github.com/en/github/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request
And your pull request is created. If the maintainer of the repository finds your changes useful they'll merge your pull request!