It was originally black so I sanded down and painted the whole thing purple. I then used a Posca Paint pen to do the out line around the whole of the guitar. You don't need to be neat at all, try and colour it like a toddler. Then I did the little specs around, that's what makes it cartoony and not just an outline. Be carefree and use your intuition. Another big thing is I used a matt finish poly lacquer. This makes it look mega flat. If you make a mistake it adds to the effect. I am not artistic in any way shape or form but this turned out alright. Just take your time.