I taught Down-stay a step at a time, and in a way he could not fail. Here's how:
Have the dog on leash at Heel. Get your Automatic Sit, then ask for a Down. Drop the belly of the leash to the ground between you and the dog and put your left foot on the leash, without making a big deal of it. Say, "Down" and pull UP on your end of the leash. (This helps the dog to lay down) Then say, "Stay," and give a Stay signal by putting the palm of your left hand flat, facing the dog, beside your hip. It does not have to be in front of his nose, don't bend over. Just make kind of a Stop sign signal over his head. He'll see it. *
Now, keeping your left foot on the leash, move your RIGHT foot away to the side, to your right. Don't lean away from the dog. Like, do the Hokey-Pokey, put your right foot out, but just to the side. Now bring your right foot back, say, "Bozo, Heel," gather up the leash and move forward. Don't snap your fingers, don't slap your leg, just walk forward and do your Heel exercises. Congrats! Your dog just did his first Down-stay!
Now do several Automatic Sits, Heels and turns before you try again. Repeat about three times. Then, move your right foot FORWARD instead of to the right. Again, do lots of other exercises before doing another Down-stay. Next, move your right foot forward, then take your left foot forward also and turn and face your dog.
ETA: After a second or two, pivot back to Heel position and heel away. In other words, go back the way you came.
(ALWAYS move your right foot forward first on a Down-stay. Use your left first first to Heel.) (Also work on being RANDOM. If you do 3 Sit-stays followed by a Down-stay and repeat that sequence three times, your dog will learn to count. Or if you do 3 left turns, a right turn and a Down-stay, five times in a row ... Yeah.)
With practice, you can start walking around your dog counter-clockwise with your hand directly over his neck until you get back to Heel position. Actually, you should have done this on Sit-stays so this would be nothing new. When you get back to Heel position, wait a second or two, foot on leash, then Heel away.
All that's left is to extend the TIME you are away (still on leash), and then gradually to extend the DISTANCE, using a longer leash. Extend one or the other, not both, at first, then combine. Going off leash, or out of sight, is another lesson.
*In competition you can't use both a verbal and a hand signal, but for training you can use both. Later, you can switch to using one or the other.