I stretch them the same way I do for guitar.
Pick a hand to start with, and extend it in front of you, palm facing out. Take each finger and bend it back, slowly. Only far enough to get a good stretching without pain. Hold for a few seconds, repeat with each finger on each hand. Also do the thumbs. For the wrists, grab all your fingers/palm and pull back, but not hard enough to make it hurt. That's how you pull something and get injured. I'd also recommend rotating your elbows and shoulders and stretching them out a bit. Whenever I seriously cube, I first stretch out my fingers, wrists, hands, elbows, shoulders, neck, and back. Even though I don't use all those parts, it helps me relax and focus.