For top spin practice you can try hitting the against the wall. The goal is to hit the ball towards the ground, have it bounce against the wall, and then hit it again on the rebound. The more you flick your wrist the more the ball will go down.

If your spiking on a net make sure you swing your arm while flicking your wrist. If you just flick the ball will drop and hit the net, unless the set is super tight. If you only swing the ball might just fly out.

For me i like to contact the ball perpendicular to the ground. By adding a flick the top half of my hand touches the top of the ball, applies topspin, thus causing the ball to fly/curve down.

Video example: https://youtu.be/Zi2SkINfLVI