1. Post your idea on the forums. Preferably post diffs if you got them. Mention your gitlab username. If everything looks fine, you'll be granted developer permissions for the project in question.
2. When you have developer privileges you can make branches, check them out, work on them locally, push them back and post merge requests.
3. Usual stuff applies: don't post unrelated issues in one merge request, try to not screw up indenting (tt-rss mostly uses tab characters), try to respect surrounding formatting style, etc.
Note: while Gitlab allows requesting access permissions for the project bots or random people kept silently spamming this feature so I had to disable it. -fox