The trash bag allows the material to slide easily over the foam and be positioned. I rip it out as much as I can when everything is where I want it. I never had a problem when I used the old covers as patterns. I tried to make one set from scratch and that didn't work out as good as I would have liked. I've never done any using leather but the only problem I could see with that is the ability to sew it. The first set I used "C" rings to attach the material to the frame on the others I used zip ties. I was going to do all of Corey's seats in fact he bought two stock front buckets to put in the back but he has decided to go with four racing seats.
If you decide to sew it yourself you can find sewing machines cheap. I used a very old Singer like this
. It only had one stitch pattern. I've seen several sewing machines on Craigslist and also there were a few at the Roberts Flee Market for $20-$40.