Assuming you have cloned
another users project, you can create a PR by creating a branch with your Unison Share handle as a prefix. This branch should contain your suggested changes. This is called a contributor branch.
@libraryAuthor/project/main> branch /@yourHandle/featureBranch
Next, push your branch to Unison Share. You'll see a link to the branch url in the UCM.
@libraryAuthor/project/@yourHandle/featureBranch> push
Head to the project's "contributions" tab click on the "submit contribution" button.

Select the branch containing your change and the target branch for your desired PR, then add a title and description of your changes.

Hit submit and you should see your PR in the list of contributions added to the main contributions page.
Finally, you can message our library authors in the #library channel in the community slack to make sure they see your PR! In the future, we'll be adding automated notifications for PRs.