Placement on your “bat”

Off the table, you can simply do bat and ball exercises, where you bounce the ball on your bat and generate different spins. This is a good way to start as you have a clear sight of where the ball is landing on your bat.
 
On the table, a good exercise is to play alternate drives and topspins. With drives, you can aim to hit the ball more in the middle of the bat. With the topspins, you can aim to hit the ball towards the top part of the bat.
 
You can do the same with pushes. Alternate between a heavy spin push (contact towards the top part of the bat) and a lighter spin push (contact towards the handle).
 
And you can do the same with serves too. Practice spinny serves with the contact towards the top of the bat and floaty serves with the contact nearer the handle.
 
All of these exercises will help you develop a better feeling of which part of the bat you are using with hitting the ball.