Every PR description has major 3 parts:-
Explain why was this PR necessary. This should clearly explain the reasons.
Explain what code changes you made which helps fix the issues or the feature. You can also explain the side effects of this change and or any assumptions
Add the ticket link in this section. Many project management portals also have status change provision from the commit messages itself. In that case we should follow the correct syntax.
If you are working on a big feature you can also create a 4th section for checklists. In this section you can track major milestones. This will help the reviewers and other programmers to understand the feature better.
eg. in case of pivotal tracker.
[deliver #123144343]
Eg PR you can checkout to understand how it is done.