Have a look at some of Josh Wright’s videos on scales on the piano. I’ll link to one and you can look for more from there. As far as thumb under goes, I wouldn’t obsess too much about it. If you keep your hand and wrist relaxed, as you increase the speed of your scales you’ll reach a point where the thumb is no longer going under very much just because the rest of the hand is moving up the scale steadily and quickly. There are all sorts of little motions involved, a little lateral bend in the wrist, a small amount of forearm rotation, but it’s hard to describe them in a way that doesn’t allow for misunderstanding or doing too much or too little of one of them. So I’d say watch some of the scale videos by Josh Wright (Graham Fitch is pretty good, too) and be patient with yourself.
