Thank you Shawn. Once I stopped struggling with this it became quite simple. I am happy to have you confirm that is really is simple!
Margaret,
Perhaps what is confusing you is the term, "stomach." Anatomically your stomach is up high in the abdomen. People often use it to mean the entire abdomen.
Specifically, the area to gently pull in on the inhale is the lower abdomen, meaning between the top of the pubic bone and the navel. The rest of your breath should fill up normally for you.
One other thought; In order to learn to create a full, natural breath, without forcing, it is helpful to watch or recall the way a baby breathes. Then visualize that easy, full body expansion and release, and model it kinesthetically for yourself.
The only difference I can see in reverse breathing is gently pulling in the lower abdomen on the inhale.
I hope that helps some. If I am adding to the confusion, let me know and I'll hush. 
Michael
