As always, start with the basics -- your World subclass and Actor classes. Actors should interact or positioned to determine when a user action is to be executed. Depending on what you want, add a timing system or some type of a scoring system, or both (controlled by the world). If you have specific problems with the code, then show codes, indicate how it is not doing what you want and what you do want.

Hey, thanks for the response! Maybe I should've been a bit more specific with my question; I see that now. I have a general idea of how all the main features, timing, scoring, and collisions are going to work, but I am having trouble with the actual charting. I was thinking of just manually spawning new notes as the song went on, but that would be a little bit troublesome to get the timing and offset exactly right. I've looked into analyzing the audio for the chart separately, in an audio editor, and using the timestamps from that to use in Greenfoot to more accurately spawn the notes in time, but I don't know if that's the best course of action. So, is there a way I could create a file that holds the data for the chart outside of Greenfoot, and be able to import it into my project for use somehow? Or do you perhaps have any ideas of what would be the best way to create the chart?