A few things to think about.
Make cardboard patterns of all the fabric. You can trace them with sharpie and then cut them out.
If you're doing a nylon outer/fleece inner, then make sure to run a candle along the edge of the nylon after cutting it to keep it from fraying.
Buckles on the straps are nice. Wide elastic is better.
Overall, it isn't that hard, but be sure to buy and know how to use a seam ripper, because you'll mess up at least a couple times if this is your first.
Fuck money, get powder.