Slylok's right on: Just do it. And keep on doing it until you're good at it. If you want something a bit more structured, though, head over to the CGTalk forums and look for their figurative and life drawing workshop threads. Loads of great advice there.

As for becoming accustomed to the tablet itself, I did it by playing Neverwinter Nights (the first one) for a few hours. The point-and-click controls for that game trained my motor skills pretty effectively. Unfortunately, a lot of games don't seem to work very well with a tablet. I tried it with Civilization V, which I thought should have worked nicely, but I was disappointed.