This year I put together a Prezi that introduced what a portrait and self portrait is and then it walked the class through each part of the face. It covered the shapes and lines of each facial feature and the location, but I didn't push real hard if they put their eyes to high or their nose wasn't quite right. Since this is the first portrait most of them ever draw, I wanted it to be a pleasant experience, not one of frustration because they couldn't get it "right".
It took us a full class to draw it in pencil and a second class to outline it with sharpie and color it in with crayon. I walked them through each step, slowly and let most finish before moving on to the next step.