Here is a tip on how to use a sock with 3 tennis balls inside. I learned this from the late and great Vic Braden. It works and is a fantastic way to practice at home.

Program the swing and then toss to the swing!

I learned this from the legendary Coach Steve Smith. The shape of the swing on a topspin ground stroke goes high-low-high and inside-out. This is a critical concept to understand from the beginning because it sets the stage for an efficient stroke. Later on it also leads to the development of lag or use of the stretch shortening cycle.