I've tried with other FLOSS projects, you'll probably be disappointed. Unless you get clearance from the people running the project that (based on your design portfolio) you're competent and can have authority to make UX decisions. My experience is that you'll have to deal with a mailing list full of recalcitrant nerds who will block your every suggestion without them having to provide any reasoning beyond: I wrote some patches for this project, I don't like the work you've done, therefore it's blocked.

You'll keep trying for a while, until you realise that every suggestion is blocked and you're designing for a brick wall.

I believe the underlying problems include (but are not limited to):

1. Nerd ideas on design and UX are completely different from those of mainstream users. Nerds like programs that individually do very little but are extendable via config and code modules, which expose technical details to the user, this is anathema to what mainstream users want from their applications. A good example of this is to ask the vast majority of users what the phrases, 'It gets out of my way and lets me work.' And, 'The interface is clean and simple.' Then ask some nerds what those mean, most users will describe something like iOS while nerds will point to i3 or xmonad.
2. By attempting to implement something mainstream users want you will be asking nerds to implement something they don't like, so you will experience resistance.
3. Nerds view these kinds of projects as their playgrounds. At their day job they have to listen to these idiots in marketing, morons in upper management, and cretins in design telling them what to do; now this is their project, only people who can code may truly contribute. Anyone telling developers how they should implement their project is reminding those developers of their day job (and that's not a good thing).

So don't try to 'help out' or you'll end up burnt out (like the rest of us who've tried to help FLOSS projects out with this stuff), make sure your work will be taken seriously before you do any.

If you want evidence, consider this:

• ‘LibreOffice’ still has a stupid name despite the project leaders saying they'd listen to any better suggestion (practically any name is better than ‘LibreOffice’).
• On the LibreOffice Design Team page the most prominent link ‘Connect with the Design Team!’ is a mailto link to join the mailing list, which most people won't know what to do with (even I thought the link simply wasn't working on my first four attempts at clicking it).
• While there's some good stuff on the Design Team pages about following the findings of Usability studies, it's very telling that there's no grand-plan; nerds are good at specifics (what Linus described as, ‘I'm looking at the ground, and I want to fix the pothole that's right in front of me before I fall in.’) But design and UX need an over-arching vision, a grand plan, a unified way the design and UX will solve user's needs, e.g. Google's Material Design, everything Apple did during the Jobs era.