We set up some poles in the arena. Just some pvc pipe and cut up garden hose about 6' long. Set them up one every 10-20 feet just layed on the ground. Start at a walk then at every "marker" speed up. At the next marker say "easy", sit back in the saddle and do half halts (squeeze and release the reins) till she slows. Do this over and over and over again until she will slow with just the word "easy" and will speed up with just a cluck or leg pressure. Repetition is key. You don't need an arena, you can do this in your yard, pasture or down a dirt lane.
I prefer teaching all of this from the saddle rather than on the ground. That's where they're going to have to know how to do it anyway