It's really easy, just clone the repository and when you are in the created folder, type flutter pub get. NOTE: To develop on/build the app, you'll need: the Flutter SDK and Your choosed platform SDK/NDK.
You should go check the Issues tab, you'll see many things to work on! On of the main problem on this app is iOS support as I don't have ANY Apple phone/computer at my home.
Just go to the issues tab and write your problem, no need to be fancy, you just need to provide enough information so that someone can see what you are talking about also please be a little bit active when someone asks you something.
This project is under the MIT license (copy, redistribution and other things permitted).