Occasionally, you'll find that the lines you're drawing will intersect, and you can't curve them properly. If this happens, create another layer on top of the one you're working on. Draw the line(s) that you need, then cut them and paste in place on your working layer. Once you've gone over the main outlines of your picture, you'll have something that looks like the image below. If you hide your base layer, you'll get a good idea of how your drawing is progressing.