To make a fused plastic tote bag, first you need to fuse your plastic. I posted a tutorial detailing this process for you. You will want to make a large sheet to make your bag out of and a smaller strip to make your straps out of if you wish to make your own straps out of plastic. I used my strap tutorial for making the straps on this bag, except that I did not need to iron these or use any interfacing. I simply folded my plastic, pinned it, and sewed it. You can see the folded straps waiting to be sewn on the right.
First trim your plastic, removing all the loose and thin edges. Aim to get yourself a perfect rectangle. Then fold the large sheet of plastic in half. Sew all the way around three sides (including the folded side), leaving the top of the bag open. I sewed up the folded side to make the bag look more uniform, with a seam on each side.
At this point you have a basic bag. If you would like to box out the bottom corners, you will fold down a triangle on each side and sew it down, measuring to make sure they are even. If you are unfamiliar with this process, here is a tutorial that explains it in detail. After sewing down the corners, trim the top of your bag (if necessary) to make it nice and straight. Then fold down the top to make a nice clean edge. You can fold it down as much or as little as you want depending on how tall you want your bag to be.
Sew this edge down all the way around the top of the bag. This is easier the bigger the bag is because this fabric isn't the most flexible stuff and can be hard to turn around on your sewing machine.
Attach your straps. I recommend using a box with an "X" in it to really secure them down. There are certainly ways of doing this that won't leave any unfinished seams and stuff, but I'm okay with a little imperfection on these sort of projects! Here is my finished bag!