You want to try to brush the ball with a thin contact like you would with the tip of a brush. Think of the topsheet at the tip of your brush, you only want to use the topsheet and a bit of the sponge, not penetrate the sponge too much and hit the blade.
You also want to pay attention to where you hit the ball, for topspin to topspin, you hit at least a bit of the top of the ball going forward. If you're topspinning/lifting backspin, more of the back of the ball with a bit of the top of the ball, going up.
Finally, if you're right handed, the contact on the paddle for a topspin is on the 1/4 that's top left, and you want to keep the ball in your rubber as long as possible but sliding it thinly toward the center, not by having it sink deeper into the sponge so that it hits the blade (that last part is what you want for a drive, hit or smash, not a topspin).
For serves, if you're right handed, the contact on the paddle is on the very top right at the tip, then you brush with the wrist.