That's a really cute bag! Of course you could needle felt the notes, staff, and time signature to the bag. Yep, you can use roving or yarn. Yarn for the long straight lines is a good idea.
To needle felt something you just poke it over and over again with a small barbed felting needle. It basicly tangles the fibers. Be sure to have foam behind it so you don't accidentaly stab yourself.
I havn't used a shape template like they're talking about. I just layout the loose fibers to see how it will look and then stab away.
Here is a picture of a bird I needle felted.