How can I easily modify and test an Android app?

There's an Android app I use that's open source. I found the GitHub repo and wanted to modify it to make a pull request for some changes I would like to see. I've seen the code, it's mostly written in Java, but I was wondering how can I set up a testing environment? As in, how can I modifiy the code, and check out how it works with the app. Do I need to compile the app into an apk, install it, and then test it for every single little change I may do? I tried Android Studio, but it's far too complex for the comparatevily tiny thing I want to do. Also, it kept giving me a bunch of errors, couldn't even compile the app. What's the best basic and easy way to do this?