I'd go onto lesson 3. It involves overlaying those drawings onto the originals and correcting/redrawing. So you'll get plenty more practice at the first two lessons and will be able to see more clearly where any shapes are going astray.
Argh, its not working! (I don't have Photoshop, btw, I have Paint Shop Pro 7)
Saw your question on John K's blog and thought I'd offer a suggestion.If you don't have Photoshop, I'd just buy some tracing paper and trace the originals. Lay them over your drawings and hold both in front of a strong light. You'll see where you're making mistakes in 2 seconds, especially in terms of proportions.