Keep these four things in mind for clean topspin contact from the baseline:
1. The distance between you and the contact point: You want to be at a good distance from the ball so you can swing effectively through it.
2. The height of the ball when you make contact: In general terms, you want to contact the ball between your hips and shoulders.
3. Where you contact the ball in the racket: You want to make contact slightly higher than the middle of the racket strings.
4. Timing: You want to contact the ball right in front of your hips so you can rotate through the ball and generate power from the whole body.