I just finished my first Osmo Masterpiece drawing, and it isn't very good. What am I doing wrong?

Absolutely nothing! It can take a little while to get used to drawing like this, and as with anything, practice makes, well... better! For most users, mastering how to interact with the app is rather quick and in 5 to 10 minutes most people start to feel a lot more comfortable. Don't get discouraged; you'll be a master in no time!