When you are working towards completing the level 2 Musical Genius life aspiration in The Sims 4, you will have to both write 4 different songs, as well as earn 500 money from licensing it. So, how do you write and license these songs in The Sims 4?
There are a total of three different skills where you can train your skill points to lisence a song; the piano, the violin, or the guitar. Any of these will do. In order to be able to write a song you will need to reach level 8 in this skill. Once you reach that skill level, simply click on the instrument and select "write", and choose a genre.
Writing a song takes a long time, and it will reset it you cancel it and redo it (in contrast to writing books which will resume from where you left off). In order to be able to finish writing a song, you should make sure that all your needs are at maximum or close to that before beginning to write a song, or else you might have to abort it, and begin from the start again.
When you sucessfully write a song, you can license it. This reuqires you to get level 9 in the instrument that you want to license. But you will get to level 9 just by writing a song anyway, so it's not like this will be a roadblock for anyone. Once you have written a song, just go to your mailbox to license it. After doing this, you will get royalties every day. These are pretty high, and you often get between 100 and 500 money each day.
You can only license a single song each week, so you have to wait until the next week to license a new song. This means that it's impossible to grind a lot of songs and get rich very easily, but you can grind a lot of songs and have a long list to sell, so that you will be able to sell one each week for a long time.
Writing all four songs for the Musical Genius level 2 takes a good while, even if you put the game at full speed. You should aim at filling all your needs, writing a song, licensing it, then filling all your needs, writing another song, and so on. Getting the 500 money from licensing songs seem to only take a few days after you licensed your first song, so don't worry too much about this.