Here are the instructions to the fused plastic bag with duct tape that I also posted on Craftster:

What you will need:
8 medium-sized plastic shopping bags (not the card-sized bags, not the giant ones, just the regular)
Duct tape (you can use plain duct tape or colored duct tape, this bag is made with black and red tape)
A stapler (I use a mini stapler)
An iron
Wax paper
Binder clips or clothespins
Scissors (just whatever ratty scissors you have. You will ruin your good crafting ones, so please... use the scissors from your junk drawer. They don't have to be sharp because the duct tape and the plastic bags cut easily.)

Step 1: Lay out one bag at a time, and flatten it, making sure all the air is out of it. Make sure the sides are folded in.

Step 2: Cut off the handles on the top and the seal from the bottom of the bag. This will create a plastic bag tube.

Step 3: Turn inside out so that the design is on the inside. This is so the design won't make fumes when you iron it. Then flatten it out again. Now your sides are not folded, it's just a flat tube.

Step 4: Repeat steps with your other bags. Stack four on top each other, so that you have two stacks of four bags. Make sure all the air is out.

Tip: For the best results, fold the four bags stacked together, in half length-wise, then in half width-wise to make a "bag stack" square and flatten out all the air. Place heavy book on top. Then go smoke a cigarette or text someone or watch the Golden Girls and come back and do the rest of the steps. (Idk why it works, but it makes a big difference in the resulting bag- the bags fuse together much better)

Step 5: Cover your ironing board with wax paper, or if you don’t want to cover it, then just make sure you have two sheets of wax paper at least 27 inches in length.

Step 6: Now take one of your “bag stacks” and put it on top of your wax paper. Cover it with two more sheets of wax paper. Make sure every little bit of plastic is covered in wax paper. It will overlap in the middle of your bag stack.

Step 7: Now it’s time to turn your “bag stack” into a “plastic sheet”. You’ll need your iron. I suggest you set it to around “Rayon”, but every iron is different, so you will likely ruin lots of “bag stacks” before you figure out the best setting for your iron. Then, just iron over the wax paper so that your bags fuse together. When you do one side, flip it over and do the other. Wait about 15 seconds, then start carefully peeling off the wax paper. Here’s what you will end up with:

Step 8: Do the same thing with your other bag stack. Then you’ll have two. A front and back for your bag. If your wax paper is still in sheets and not all torn up, save them.

Step 9: We’re going to make the hem at the top now. Turn your first sheet horizontally. Fold down the top, about an inch or so, to make a “hem”. Use binder clips to keep it in place.

Step 10: With your stapler, staple the hem down, like if you were sewing. The front of the staples are on the front of the bag, so as in the picutre, the bag of the staples are on the "wrong side" of your plastic fabric.

Step 11: Now take your duct tape and cover the line of staples, then fold it over the top so it covers the back of the staples, where they would stick you.

Step 12: Make a hem on your other sheet. Step 13: Fit one sheet on top of another and use binder clips to “pin” them together. It’ll keep them straight while you staple them together. Clothespins also work, so do paperclips, but I recommend clothespins or binder clips.

Step 13: Fit one sheet on top of another and use binder clips to “pin” them together. It’ll keep them straight while you staple them. Clothespins also work, so do paperclips, but I recommend clothespins or binder clips.

Step 14: Start on either side and staple the sides together. Then do the bottom and the other side.

Step 15: Cover the staples with duct tape like you did the hem.

Step 16: Now for the straps. You’ll need your wax paper that you saved. (or cut yourself 2 more sheets of wax paper approx 27 inches in length or whatever you want, longer or shorter) Fold up the bottom 2 inches. Keep folding over and over and over until you have a two inch strap. Put one staple 2 inches from each end and one in the middle. Do this with your other piece of wax paper.

Step 17: Cover the straps with duct tape. Do it however you want, but I like to start on one side and fold it over like I do the hems, because folds on the two sides makes for a stronger strap.

Step 18: To attach the straps to the bag, turn the bag back inside out (unless you never turned it right side out) and duct tape them to the inside of your bag. I have made like 473985739857 of these bags, perfecting the method, I guess, and I carry college text books in my bags, and I have never had a strap break. The duct tape is surprisingly strong and adheres well to the plastic.

Tip: For added strength, you can be crazy like me and hot-glue them. Hot glue takes some practice, but I promise you, it can be done, if you're careful. Use too much and you'll (duh) melt a hole into your bag. But a thin layer of hot glue to adhere the straps before duct-taping them, goes a long way in terms of strap durability.

Step 19: Okay, your straps are on, you're done. Some of my bags have designs on them. Stay tuned for pics and instructions on how to add some designs with your duct tape, some reused cardboard, and your hot glue gun.