If you are anything like me, when you go shopping and that store has a particularly neat looking plastic bag, you save it in hopes of finding something cool to do with it eventually.
Without further ado, lets get started on this.
The things you will need are as follows:

Several plastic bags (In my case, I'm using the ones from World Market.)
Bussiness cards, credit cards, and dollar bills, for sizing.
An Iron and Ironing board.
Bobby pins (it makes it easier to hold your bags together while sewing.
A sewing machine or a needle and thread.
A rotary cutter and cutting mat, or scissors, but a rotary cutter makes this whole process 10x easier.
An old insert from another wallet. That plastic flap thing that grandmas and grandpas keep photos of their grandkids in.
Some entertainment while you work. (I would just whistle while I worked, but I sadly, can not whistle, so some stand-up comedy via netflix or the like will have to for me.)

Step 1: The Bags.

So the first thing you need to do is find some neat-o looking bags. Or just plain ones. Generlly thicker bags work better, that or if you are using the paper thin ones from wal-mart, I recommend layering several of them in a stack for irnoning, so that way your end result isn't a sad flimsy wallet.

After you have picked out your bags you will need a few things to use as a guide for measuring, Keep in mind that when you cut your plastic, do NOT cut it the same size as your business card or dollar or whatever. This results in a wallet made for a parallel universe that has tiny dollars and less then standard size business cards. Seam allowance guys. It's a thing. (Can you totally tell I messed this up several times when making one of these?) I've since made about four good wallets, but they do get worn out over time.

Step 2: Cutting Your Plastic and Ironing.

WARNING: Please, keep in mind, you are using a hot iron on plastic and paper. Do this in a well ventilated area, and keep the iron on a very very low heat. Also, kiddos, Get an adult to do this for you. It may take some convincing after you walk up to them with plastic and say "Hey can you iron this please?

Now that we are past all the safety business. We can get to cutting. Which, oh yeah, be careful with that too!
Basically you lay out your plastic bag and lay your measuring units on top of it. Make sure it's a fancy part of the bag that you like.
Carefully cut. I will list below all the pieces you will need for a basic fold over wallet with three pockets inside it.

Depending upon your thickness of plastic you may need more or less of said plastic.

Outer Wallet Panel: 4 cuts of plastic
Inner Wallet Panel: x4
Pocket 1: (For inserting your plastic flap that holds your license and such)x4
Pocket 2: I use this one for business cards and such.x4
Pocket 3: This is my main pocket, I use it for my debit card and frequently used rewards cards, etc. x4

After you have cut all your sections and stacked them, secure with bobby pins if need be, but remove them before ironing!
When stacking keep in mind, If your plastic has any type of coloring or logo or anything, you need to either:
A: Lay the pieces ink side in.
or B: Lay them ink side up (If it's a logo and you want it the right way) then lay clear plastic over that.
It is very important that you never iron stuff with the ink facing out, it will smear and make a mess and likely ruin your wallet and nanna's iron or board. It's a mess.

Now, with your iron on a low setting, place a piece of paper down, and then your first stack of plastic on top, then lay another piece of paper over that, basically a sandwich where the paper is the bread and the plastic is the sad excuse for meat.
Run your iron over the paper a few times, do not sit the iron on the paper for too long, this can result in burning or warping your plastic.
Lift up a corner of the paper, if you have one solid layer of plastic, you are golden, if not, keep ironing a bit.

Step 3: Sew It Up!

The first part you need to sew are your pockets, place them onto your inner wallet piece, and sew. Be sure to measure once more and make certain your placement is correct and how you want it before you sew. If it helps look at a normal wallet for guidance.

After your pockets are sewn to your inner piece, you should now have two pieces, one with pockets sewn to it, and one that does not. Place these two together, make sure your designs are facing whichever way you want them, and then sew up both sides and along the bottom.

Step 4: Oh Hey! a Wallet!

Bask in the glory of your beautiful eco-friendly wallet and then go make some for your friends.
Don't stop at wallets either, now that we can fuse plastic we hold the power of making lots of things from plastic!! Tablet covers! Satchels! Murses! Purses! Backpacks! The possibilities are nearly endless!