For this bag, I've made it 2 sheets of HDPE high. Sew together two sheets into a single panel. Fold one over on itself for a nicer looking seam on the outside. I double stitched all my seams for added strength

You'll need 3 10"x20" panels assembled. Split one in half long ways into 2 5"x20" panels for the sides of the bag, and reserve a 10"x5" piece of HDPE for the bottom. Build up some more if you want to add any pockets or other embellishments to the bag.

If you don't have a sewing machine, there's always hand stitching - but there may be another option.

Remember that HDPE is a thermoplastic, and that means we can fuse it. Using the edge of your iron, a careful and consistent application of heat to each seam should fuse them all together into what would effectively be one sheet. Remember, these bags were destined for the trash, and even fused together can still be recycled if it doesn't work. Experimentation is half the fun!

Another trick to keep in mind when we begin to use nylon webbing in the next steps is to melt the ends. Carefully (and if you aren't one, under the supervision of an adult) use a lighter or candle to singe the ends so they don't fray or unravel. Do this as quickly as you can when you cut it. It doesn't take much for it to start to pull and fray.

If I started over from scratch, I'd try and make a larger pack. This one certainly works, but after stitching was a little smaller than I wanted. Consider the final size of your bag when cutting your panels, knowing you'll lose 2-3 inches of height from the closure, and another 1/2-1 inch on all sides from seams.