I'm very new with Skritter, and having a hard time understanding how to practice one lesson from a textbook. If I was starting with Lesson 1 it would be fairly simple, but I'm on Lesson 19 of the text. How do I isolate only the one lesson to study, without adding any other characters, and without reviewing any other characters? Thanks a lot -- I'm really excited about Skritter, just trying to tweak it to my study needs.