Seeing the Instructables backpack challenge, along with my friend Jessie Uyeda's grocery bag upcycling video were the inspiration for this project.

You could certainly make this backpack exactly as is, but of course I encourage you to build a bag that works for you. There were some things I'd change if I made it again, and I'll detail that in this guide.

You will need:

Approximately 30 used grocery bags. (you could use new, but it defeats the purpose of this project)
A clothes iron or heat press (like would be used for heat transfer vinyl or screen print)
Parchment paper or silicone mats for your heat press/iron
Scissors or rotary cutter and self-healing mat
A sewing machine (but you may be able to even get away without one - details in the instructions!)
Some nylon or polyester thread
Approximately 3 yards of 1" nylon webbing
6 1" D rings

Step 1: Fusing Grocery Bags Into HDPE Sheets

This is a trick I heard about a while back and saw in practice in Jessie Uyeda's video, linked here.

Most common grocery bags are made of polyethylene, a thermoplastic. What does that mean though? It means that when heated to an appropriate temperature, the plastic materials are malleable, will bond/fuse together and be used to form shapes or in a thermoform/vacuform machine. Fused together, you get a sheet of High Density Polyethylene or HDPE.

Each sheet I made was comprised of 3 bags (6 layers) I put the bags inside each other to help them keep together and did my best to flatten it out and keep them even.

Now I used a heat press. I set the temperature to 275 degrees, and sandwiched between the silicone sheets on it I did 35 seconds on the first side, and 30 seconds on the other side.

You don't need a heat press. Sandwich your bags between some parchment paper and set your iron to the cotton/highest setting and keep it moving over the bags until they are fused into a single, durable, tough sheet of HDPE. This may take some time. Experiment! As long as you keep the iron moving there's no risk. I recommend using a timer/stopwatch so you can get repeatable results once you have your method dialed in.

Step 2: Cut the Sheets to Size.

After your sheets are fused, you may have some spots around the edges that are thinner or less consistent. We're going to trim all the edges off to have consistent sheets we can work with. I got approximately 10"x10" sheets out of each bag. The self-healing mat and rotary cutter made this easy, but a pair of scissors will certainly work just fine.

Now we have our raw materials!

Step 3: Build Our Panels for the Bag.

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.

Step 4: Assemble the Bag!

Now we're going to finally see our bag come together.

I've made a few bags now of varying shapes and sizes and the biggest lesson I learned from this is it's much easier to work with a flat piece of fabric than one that's assembled and sewn. Like many other "maker" disciplines, it's important to consider your order of operations to assemble the bag. My photos don't necessarily show this because this was a learning experience for me.

Using the drawing shown here in this step, sew your panels into a large flat panel resembling this. Notice where I showed the seams as ------- we have our 5 panels for the bag - side, back, side, front and bottom.
1. Decide now which side will be the inside of the bag and sew/bond your material accordingly. Do all of your stitching on the "inside" side of the bag if you don't want any visible stitching on the outside.
2. You can attempt to carefully fuse the panels together with the edge of your iron. As long as you get good layer bonding, this will be as strong or maybe even stronger than sewing. Hand sewing is always an option too.
Attach your nylon webbing - one strip along the top with 2 d rings attached to the middle of the side panel shown in the sketch, two 36" straps attached for our bag straps, and two small loops for your D rings. the D rings should be doubled up for each of these - so yes, 6 D rings in total. This allows us to create an adjustable pull strap. The top strap should have about 6" hanging loose on the side panel shown in the far right of the sketch, this will be used as our closure.
1. These can all be hand sewn on if you don't have a sewing machine. You cannot reliably fuse nylon.
2. Sewing through multiple layers of nylon webbing can be tricky with some sewing machines. I recommend trying a denim or "heavy duty" needle to avoid binding or "bird nesting".
If you want to sew any pockets or extras on the bag, now is the time to do that. I had an old laptop sleeve I wasn't using anymore that I sewed into the bag. Something I did learn is that the neoprene-like material it was made from was very finicky on the sewing machine. It would skip stitches if I was going slow, but worked fine going quickly. Some experimentation may be needed depending on what you're doing.
Time to bring it all together! Remember which side will be the inside of the bag. We're going to close the bag up inside-out, then turn it outside-in when we're done.
1. Using the sketch as reference, bond or sew the open end of the front panel on the far left to the open end of the side panel shown on the far right.
2. Attach the 3 open edges of the bottom panel to their respective side panels and the front panel.
Turn the bag inside out! Almost done!

Step 5: Straps and Closures - We're Finished!

With our D rings, we can run the straps through them as shown to have adjustable straps for the back.

As for the bag closure, fold the top over itself 2-3 times and buckle it the same was as we did with the back straps. This also serves as a top handle for the bag. This type of closure is typical for dry bags used for boating and marine use. And since HDPE is waterproof and super durable, your bag should be too!

Thanks for checking out my instructable, and please let me know what you think in the comments! Did you make one? Did you do anything differently? What did you learn from the process?

Thanks again!