It is pretty simple. You tie a string between two sticks and put them several feet from the plate. The pitcher's goal is for the ball to go over the string and hit just behind the plate.
My DD played D1 ball, and we used a tall plastic trash can. We laid a plastic pickle bucket on its side just behind the plate, with the mouth of the pickle bucket facing the pitcher. The goal was to throw the ball over the trash can and into the bucket. We did it as a game. We had about 100 softballs. So, she would start with the trash can perhaps 25 feet from the plate. Everytime she got the ball over the trash can and into the bucket, we moved the trash can toward the plate a couple of feet.
The pitcher has to develop a feel for the speed, spin and release angle for a drop ball. Personally, I think using simply a string at a fixed position doesn't challenge the pitcher enough.
I use 2 fiberglass poles, that I think contractors use. I bought them at Lowes. They are flexible, but pipes in cones would also work. I think that my string is about 2 feet off the ground and 2 feet from the plate. You can play around with that part.