Instead of jumping right in and cutting up all my (expensive) leather, I thought it would be a good idea to make a prototype to make sure all my measuring was correct and that everything looked right when it was attached where it was supposed to be.
I am SO GLAD I did that, because I realized (1) one of the pieces needed to be eight inches longer because I should attach it to the underbust strap for stability, and (2) I need TWO overarm straps, or the back pieces have nothing to attach to on one side. So this was a very valuable exercise and I should always do it when working with expensive-to-replace materials.
So this is part of my prototype, which I made out of the same packaging foam that I used to cover the lights for Iron Man. (That stuff came in VERY handy this past week!) I labelled everything with a Sharpie so I'd know where each piece went and how long I cut it (and if I needed to add notes like "add two inches" or something, I could write it right on there). Then I taped everything together, including the (not yet spray-painted) O-rings where needed. I made marks on the tape where I'd be adding snaps or elastic or Velcro so I'd know what I could and couldn't un-tape to get out of it. (I would be the person who makes an absolutely perfect piece and then realizes she can't put it on or take it off.) And from doing the prototype, I discovered that there's one less place I need to add a fastener (it will slip over my head after I undo the collar piece), which is awesome, because I expect that to be quite a pain.
I've cut the leather now (and I have ONE spare belt blank in case I need to re-cut something) and painted the edges red, so everybody cross your fingers and hope that it all fits together the way it did in my prototype!