The Sims 4 allows your musically inclined sims to write some songs of their very own. It’s very simple to get started, but takes a really long time. This is basically the same thing as writing a novel, for example. You can be better at it if your sim has musical traits, and you can replay the song your sim writes.
First, get to level eight in the instrument you’re going to use. Once you do, you’ll unlock the ability to “Write Song” with that instrument. Again, it will take your sim awhile to do this, but that’s all there is to it!
If you stop your sim mid-song writing, all you need to do to continue is to use the new book that will appear in your sim’s inventory. Congratulations, your sim will be a music writing savant in no time!