Sorry yeah I've gotten so used to building android apps its probably more complicated than I think.
Open android studio. Select new project from version control. Enter the url of the github and click enter. This will download the source code and open it to view/edit. Android studio should then prepare dependencies etc, it will say something like "building gradle" in the bottom corner. Wait for that to finish then in the toolbar at the top of the screen there is a dropdown labeled build which has the different build options. There should be one called "build apk" this will build the app.
Now that I've written this out it does seem more complicated that I thought.