I did it once, although I don't know if I did it technically "right". What I did was trim my batting and front flush by sliding my rotary mat over the backing material and cutting only the top and batting. Next I trimmed my backing 2" out from my top - I have a 2" wide 18" ruler I used for this. Next I folded my backing corners in toward the top and pressed, then folded my sides in till they just touched the top and pressed. I may have done that twice - its been years and I can't remember for sure. Finally I wrapped the backing around the front edge and used a hem stitch on my machine to tack it down, although if I had it to do over again I would do it by hand.
Like I said before though, nobody taught me how to do this so this might not be the right way, I just was trying something new to see how it would work. You may find better instructions or a demo somewhere else. You might try You Tube