-1. Post your idea on the forums. Preferably post diffs if you got them. Mention your gitlab username.
-2. If your idea was accepted, request developer permissions for the project via Gitlab.
-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.
-4. That's it.