I start the lesson by talking about Picasso and his use of musical instruments as the subject for many of his paintings. I also talk about his "blue period", and Cubism.
Next I show them an actual guitar and talk about all the types of lines and how they can go about drawing it. Usually I tell them to simplfy it unless I am teaching an older group of kids and plan on doing the lesson in 2 or 3 sessions. After they do a basic drawing on a practice sheet, they re-draw it on a sheet of watercolor paper. Then they use a ruler to break up the background of the guitar,making several different geometric shapes. (The more shapes they have the longer the project will take, so adjust it according to your class time.) I also have them section off their guitars in a few shapes. If it is a younger group I have them outline their pencil drawing with Sharpie at this point.
Next I give them a choice of three different blues. They start with one blue filing in all the shapes they want to be that color. Then they add a little bit of white and fill in all the shapes they want with that color. They can get about four or five different values out of one blue (or more) by continuing to add a little white. I have them continue the same process with the other two blues.When they are finished they go over the lines with Sharpie.