Because I've had a few people ask for help with drawing, this is a tutorial on how I learned to draw them.
If you're using pencil and paper, the Blue lines should be light pencil marks, the black lines should be darker pencil marks that you will trace over in pen once satisfied.
If you're drawing digitally, make sure the blue lines are on a different layer, under your black lines, then you can easily delete them once you're satisfied with your line-work.

Hope this helps, And remember: PRACTICE!