This is how I was taught to teach my puppy to "stay." I'd still recommend a class b/c they will be able to show you how to do this better than I can explain it. Of course during this exercise, the puppy should have his leash and whatever training collar he uses on.

1. Have puppy sit on your left facing same direction as you are.
2. Put an open hand in front of his nose and say, "Stay"
3. Pivot in front of dog so his nose is almost touching your knees as you face him.
4. At first only stand there very briefly (1-3 seconds) then pivot back so you and dog are both facing same direction.
5. Give the release word and motion (I say, "OK" and wave my hand). This is very important b/c the dog must learn to stay until he hears the release word.
6. Reward dog

I wouldn't step any further back for a few days until the puppy has got it. After a few days, when you pivot in front of the dog, leave about 6 inches between you and the dog. Keep doing this and very gradually (probably over the course of weeks) you can increase the distance between you and the dog. If the dog isn't staying, you've moved too far away and need to stand closer to the dog.

Finally, while teaching "stay" make sure you return to your position and release him from there. This makes sure he knows he is rewarded for waiting for you and not coming to you.

As you can see, this takes a lot of time and it really helps to have a pro demonstrate this and critique what you are doing right/wrong.