The motion itself is very simple, flex and relax your forearm, breathing out on contraction and in on relaxation.  You could bring the weight down two ways; by holding the dumbbell in a fist or by extending your fingers and allowing the weight to roll down to their tips (then bringing the dumbbell up with your fingers into a fist and lifting it for a full contraction).  Rolling the weight down to the tips of your fingers is a great way to extend the limited range of motion of the exercise.  You can perform the seated dumbbell wrist curls with one arm at a time or with both arms simultaneously.

If you choose to perform the seated dumbbell wrist curls on your knees, avoid cheating by bouncing your hand up with the knee or by swinging the weight.  Control both the contraction and the relaxation of the flexors.  Keep your wrist completely removed from the support (bench or knees) to allow the greatest range of motion and to keep the pressure on the flexors throughout the exercise.