This started out with me thinking of an easy, cheap, and quick way to make a thin and good looking wallet. I looked around my room, and realized that card sleeves (see the pictures later on if you don't know what I'm talking about) are the perfect size for bills.

...Wait for it...
Remember that this is in the Pocket-Sized Contest, so vote for what you like.

Step 1: Materials List

Materials:
-Packaging Tape (or any other kind of tape if you don't care about the wallet being see through)
-Scissors or an xacto blade
-Card sleeves

Step 2: Cut Out Two Card Sleeves From the Sheet

Some may have pre-cut card sleeves, and if you do, you can skip this step.

Simply cut around the borders of two of the card sheets.

Note that by cutting out a sleeve, you make the middle row of sleeves unusable, as there is only one border in between the two.

Step 3: Cut One of the Card Sleeves

Now you need to cut a hole so that you can attach one sleeve to the other with tape.

Cut a rectangular hole in one side of one of the sleeves so that a piece of tape can go on the inside of the sleeve and part of it will stick to the other sleeve when it is attached.
Make sure to put a piece of tape up to the sleeve and check the width so that the tape will fit properly.

Step 4: Attach the Other Sleeve

Attach a piece of tape on the inside of the sleeve you just cut, and attach it to the other sleeve. Press hard so they stick together well, and you now have a new wallet.

Tips:
If your tape and your card sleeves don't stick well, try cutting out both sleeves so that the connection is between two pieces of tape instead of with the plastic.

When cutting the hole for the tape, use an xacto blade to make better looking cuts.

If you treat your wallets really roughly or if your tape doesn't stick as well as mine did, attach tape on three of the edges of the sleeves to help keep them together. And/or you can cut two holes in one sleeve so that you have two pieces of tape connecting the two sleeves.
One piece of tape has held up for me.

Also remember that if you don't carry around a lot of cash, it's easy to just cut out one sleeve and use that, although I found that to be too small of a wallet for me.
And also remember that you can have even more than two sleeves if you need more space.