I train it the way Rachel does.
And if you're not 100% sure that your dog will remain in position once you get beyond leash length, you can tether your dog to something fixed, like a piece of furniture. If he does try to get up and walk toward you, the tether reminds him "oops! I'm not supposed to do that!"
I don't like doing that (because he's already OUT of position before he's corrected) but it's an option.
Also, one thing I hope you realize is that your eyes will draw your dog toward you like a magnet. One of the biggest errors I see new students and puppy owners make is that they put their pup in a position, and they back up, staring at their dog. The dog can't resist, and comes toward them. Also the motion of backing away indicates to your dog that you're not really confident in his ability to stay in position. That makes him nervous and he's more inclined to move to you.
So, if you're still working on the leash, put him in position, take a few steps back, and look away. Keep an eye on him from your peripheral vision. If he starts to move out of position, and you've caught it fast enough, then give the verbal correction (oops!) or if he's completely disregarded his position and stood up (ah-ah). Or as Val says, the "look" (if you have one) is good too.
But once you get beyond leash length, just give the command briskly, like you KNOW he will comply, and walk away confidently. Glance quickly over your shoulder to make sure he's not getting out of position. But avoid backing up slowly, staring at your dog!