Step 2: cut and trim your pieces
Note on water bottle selection:
You want to get the thinnest water bottles you can. I used Costco brand water.(cause that's what my mom drinks, despite how many times I tell her to drink tap water).
I tried using soda bottles, (coke, pepsi) and they are too thick. They melt fine, but they are hard to sew. You can sew by hand if you push the needle through with some plyers, but it's a pain in the bum. You can sew with your machine if the plastic is thin enough.
Trim you pieces of plastic, so the edges are smooth and straight.
Cut you pieces of fabric.
You can make the bag any size and style you want.
I made two square panels, plus a botton panel of plastic.
Just sew as you normally would with any fabric. Right sides together, back stitch at the beginning and end.
I sewed both panels first, then sewed them together, then sewed on the bottom. Just go slow and steady!
When sewing the plastic, you might want to wear safety glasses. If the plastic is not thin enough, or you hit a bump, there's a good chance the needle will break, and there's a good chance the little bit of needle will hit you in the face. (trust me, i know)