Putting on the Brakes
Have the pitcher throw her fastball, the pitch she throws with her fastball grip. Have the pitcher gently put on the brakes at the end of the wind-up so the ball only returns as far as the 60 percent mark. Have her practice that until she can consistently throw it to where the ball comes off the wall and lands on, or very near, the 60 percent line. It will not take long for her to figure this out, as she will catch on to this quickly. It is important to continue keeping the wind-up at 100 percent full speed, but gently slow it down at the last moment.
Once she is consistent at 60 percent, have her do the same thing and have the ball return to the 80 percent mark by again applying the brakes to the last bit of the wind-up. Have her practice throwing at that speed until she can consistently come down on, or very near, the 80 percent line.
Then have her throw to the different lines on command. Have her throw a pitch at 100 percent, then 60 percent, 100 percent, 80 percent, 80 percent, 60 percent, 60 percent, 100 percent, 60 percent, etc. Once she can do this and come pretty close to the correct speed/line, she is ready to try it on the batters.
Changing Things Up
When one pitch is a different rate of speed then the last pitch - and there are seldom two pitches in a row the same speed - the batters have greater difficulty deciding when to swing. They will have to depend on watching the ball travel from the pitcher's hand to determine exactly how fast it is coming. Forcing them to do that reduces the amount of time they have available to around 40 percent of normal.
You can also divide the distance between the 40 percent and 100 percent lines into two equal parts and establish 40 percent, 70 percent and 100 percent distances. This will give the pitcher two speeds available for any particular pitch. Again, the 100 percent distance will be accurate, the others are estimated distances.