It isn't so much a heels-hip-shoulder alignment, as keeping your center of gravity over the stirrups. That CAN be done in some riding with a heel-hip-shoulder alignment, but not for a forward seat:
That is a picture of Gen Patton taking a jump, but if you draw a straight line up from the stirrups, you'll see roughly equal weight in front and in back of the stirrup. In this sort of riding, your body unfolds as well as your legs during the upward thrust (which comes from the horse, not from trying to stand up in the stirrups).
The picture below shows someone who is NOT a good rider (me), trying to post lightly in an Aussie style saddle:
If you draw a vertical line up from the stirrup, you'll see I'm a little too far forward and that has driven my stirrups a little behind the stirrup bars. As a rule, the stirrup straps ought to stay vertical and the stirrups not move...and no, that ideal doesn't describe my riding.
But I'm not grossly off, either.
"So how in the world do I hold my feet in the proper position so that when I try to straighten my leg to go up, I don't either fall backwards or push my feet out forwards?"
First, don't straighten your leg to go up. Let your leg unfold as the horse's movement pushes you slightly up. As best as I can figure it, you want gravity on your side - and tension in the leg muscle pushing against the stirrups to raise you prevents gravity from being your friend.
When your rump is in the lowest part of the saddle, where are the stirrup straps & stirrups, assuming the straps are vertical? If it is under your hip, then you can keep your back vertical while going mostly straight up. If your saddle is designed to put them a little forward (BTW - I read recently that a British Cavalry manual from the early 1800s said a plumb line dropped from the soldier's shoulder should land an inch behind his heel...just an odd historical fact), then you'll need to lean a little to keep your balance over the stirrups. How much depends on you, the horse & the saddle.
You might try riding at a walk and trot while standing in the stirrups. That will help you develop a feel for the balance point so you can unfold legs (and body if needed) above that balance point.
And remember - the picture proves this advice comes from someone who is NOT an expert!