When pursuing the Musical Genius aspiration in the Sims 4, you will have to mentor other sims in a musical instrument for a total of three hours. This task in itself might not look that difficult, but it can actually be very annoying to get this done if you don't know what exactly you are going to do.
Firstly, in order to mentor another sim you will need to reach level 10 in that skill. For the Musical Genius aspiration you can mentor in either the violin, the piano or the guitar. You only need one of them, so don't bother leveling all three of them to skill level 10.
After you have reached level 10, you need to buy a trait from the aspiration trait store. This cost 1,000 trait points, so you will have to save up a little while to get this. This trait will allow your sim to mentor other sims in any skill where they have level 10 (including all the non-musical skills).
So, now you have the skill and the trait, and the next step is to find somebody who is actually using the instrument. Unfortunately you cannot just click on a sim and mentor them; you need to click on them and select "Mentor" while they are actually using the instrument. This is very rare to come across naturally (they are mostly untouched even in the bars and lounges in the game), but it's super easy if you play a household with two or more sims. Just select one sim to play it, and another to mentor that sim.
But how do you mentor another sim in an instrument if you play a single sim household? The solution is to click on the instrument and select "Hire guitarist/violinist/pianist". This cost 100 money, but will soon get you a player on the instrument. Once this hired help begins playing, click on them and choose "Mentor", and you will begin to get credit towards it. It will only take a single hire to get the "Mentor for 3 hours" aspiration completed.