Get the code downloaded somewhere and open a command prompt / terminal in that directory
Run "gradlew idea" if you'd like to use IntelliJ as your IDE, then load from the generated projects files, or even just use "gradlew run" to run the game from source via command line
Edit code to your heart's content in IntelliJ or elsewhere
Run either using the IDE's run options (created automatically for you in IntelliJ) or from the command line with "gradlew run" again
Anything more than that need some more details on what you're wanting to do