This advice is pretty universal. First things first, install the app and get familiar with. Then write down all the ways you believe that the app could improve. Then break the list down further to figure out what are the goals and how they’re obtainable. Check those new list items against the Material You guidelines. After that, your list should be smaller, items that can be grouped together, should be. Figure out if you need to make a mockup to help illustrate your advice. And then create issues on the project GitHub, the more the better. Afterwards, shoot the developer a private message or email and explain that you’re a UX designer and you’re hoping that you can contribute in a meaningful ongoing manner.