The simplest ways to contribute to LibreOffice are to submit bugs and enhancement requests to our bugtracker or to review and comment on the reports of others. If you think something in LibreOffice breaks a UX principle or guideline, it's a UX bug. Learn more in the blog post about Tickets on Behalf of UX, or in general from Fabiana Simoes's GUADEC presentation about How to not report your UX bug.
We have tasks for everyone from the very beginner to advanced developers. LibreOffice calls simple tasks easyhack, and together with the keyword “skilldesign” you may search the bugtracker.
A number of tasks are not listed in the bugtracker for some reason. For instance, create a new page at the wiki, draft a guideline, or create a mockup are better managed outside Bugzilla. For this purpose we use this wiki. Many tasks can be done by everyone, some require creativity, and other need more usability knowledge.
Bug reports and feature requests needing UX evaluation are marked with the keyword needsUXEval. Feel free to add your comments and even sketches to the reports. Another approach is to look at unconfirmed feature requests with a critical eye. One task of the UX team is to help decide on what features should not be added into the core application.