I cut a couple pieces of big gray trash bag to cover mine because I wanted it gray. Then I cut up some of my colored bags in weird pieces and laid them on top. This is going to be the outside of a tote bag that I intend to make from this fabric, so Step 4: Decorate your "fabric" with fun colors and stuff. I wanted it to be colorful and fun, and trust me, you can't really mess this part up. Once you iron it, it always looks cool. I just cut random shapes.
You don't have to do this, but I cover my decorated parts with a layer of clear plastic. This is just because different types of plastic melt and fuse differently and I want to make sure to lock everything in really well. By now I have probably 10-ish layers of plastic. You can do less and you can do more depending on the plastic. Thicker bags take less, cheaper and flimsier bags take more. There's no magic formula. Step 5: Cover this with more parchment paper.
Step 6: Iron your plastic on a high setting (no steam), hovering over each section for a few seconds. I go over it several times moving my iron slowly to make sure that it fuses really well. Your plastic will shrink down while you do this, so be prepared for that! That's why I make a big ol' piece to start with. Make sure you flip it and iron it on the other side as well.
Tada! Here is my finished piece of fused plastic! I went back and fused another piece that I will use to make straps for the bag that this is going to be. To the right is a bag I made earlier from this same process. This bag is sturdy and durable, can be fairly easily wiped clean, and holds its shape so that it will stand up even when empty, which I think it convenient!
I will post a tutorial on how to sew the fused plastic into a tote bag soon. This is one of my favorite recycling projects!