Please forgive the total lack of technical terms!!
Starting from the wrist I make a tube, stopping at the bottom of my thumb. I then with 2 bits of wool pulled through, mark the gap for my thumb, back and front and stop then going around, but go back and forward. When the length is correct, ie when the wrist warmer is the right length, stop. Put it on and work out where you have to pick up the stitches on the side from the top on the palm side. and work a small band until it is long enough to join back to front. Then if you want you can pick up stitches and crochet the thumb around.
Here is a fairly dreadful diagram, (drawing is not one of my talents!!)
Good luck and please let me know if this was helpful/unhelpful.