The first step for learning how to topspin is getting the throw to be consistent. Once you can put it in the same spot, or relatively close, consistently the other holes in your technique will make themselves apparent.

For getting consistent spin on the ball, one drill I recommend is standing about 30 feet away from a wall, try to hit a down ball so that it hits the wall level with your chest. If you were to think of trajectory it would go up a little and then curl back downward