Use the instrument and read the musical bar like this:

Play the notes from left to right. Everytime you get the Note right the light below will glow.

Using your picture as example the first note is on the 3rd line. Just try out wich key plays the right note.

The next note is above by two spaces and one line so 3 notes higher -> 3 keys further

I hope this helps!

If you still have trouble just destroy the walls on the side/back :)