There is no shortage of wallets posted here, but when I lost my wallet a couple of years ago, none of them did it for me, so I made my own. You'll need some sewing skills for this one, but the result is a pro-ish looking wallet that, depending on what materials you use, will last a long time. For materials, Tyvek envelopes are good, but anything with good tear and abrasion resistance will do (e.g. mylar, airline baggage tickets, your old parachute pants - you get the idea). For mine, I used a Tyvek envelope for the interior and the packaging from some bike tires for the exterior.

Also, my first version of this (not posted) tore a little too easily and I could only patch it so much, so I added a fabric liner to this version to give it some added resilience.

Step 1: Tools and Materials

You'll need some tools:

Sewing Machine (you could also sew by hand)
A blindstitch/edgestich presser foot for your machine (optional, but will make your stitching much straighter)
Spray adhesive
Scissors
Ruler

And some materials:

Exterior and interior (Tyvek or some other tear resistant material)
Silk
Spray adhesive
Double fold bias tape (go to the fabric store for this)
Thread

Step 2: Interior Card Pockets

Start with the inside first, because it will dictate how large the exterior should be. I'm using a used FedEx envelope and I'm taking advantage of the fold. The piece you cut needs to have enough space for 2 credit cards, 1/4" spacer for the fold, and about 1/8" on either side to catch the stitches.

Cut a 7 3/8" x 2" rectangle off one side of the envelope, including the fold, so that if you unfolded it you would have a 7 3/8" x 4" piece.
Cut 3/4" off one of the folds lengthwise, so one fold is 3/4" taller than the other
Cut another 7 3/8" x 2 1/8" single piece off the envelope. This piece will be sewn on to the first to create the second row of interior pockets
Cut three sections of about 8 1/2" of bias tape. The measurement doesn't have to be exact, but you want it to hang over the edge of your material so that the sewing machine catches it.
Position the bias tape over the top edge of each of the interior pockets
Adjust your edgestitch foot so that the stitch falls in the middle of the tape (i.e. a 1/8" edge for 1/4" tape)
Sew the tape on to the pockets.

Step 3: Sew the Pockets Together

The big piece with the fold will make the lower pocket. You'll sew the smaller piece on to the larger piece to make the upper pocket.

Fold the larger piece along its existing edge and measure the distance between the two rows of bias tape
Position the smaller piece behind the larger such that the three rows of tape are equidistant, then either mark the position or affix it with pins or tape
Unfold the bottom, larger piece and sew the smaller piece to it leaving an 1/8" trim*
Marvel at your new interior pockets

* You can either sew it directly which is easier, or flip the smaller piece down, sew it and fold it back up. The latter is more difficult but cleaner and is what is shown in the pictures. Keep in mind that if you do that, you'll have to position the smaller piece 1/4" higher than you would otherwise to account for the fact that the trim will be on the inside of the pocket instead of the outside.

Step 4: Sew the Pockets Edges and Center

To keep the cards in place, you have to sew the pockets together:

Cut the excess bias tape off
Sew the sides of the pockets together leaving about 1/8" trim
Position two credit cards on the pockets and mark a 1/4" span in the center along the bottom of the pockets
Sew straight up from the first mark to just above the second line of bias tape, turn the pockets 90° and sew 1/4" along the top of the tape, then turn the pockets again 90° and sew to the other mark.

Note: to make a 90 degree turn while sewing, position the sewing needle down (to keep the material in place), pick up the presser foot, rotate the fabric, then put the foot back down

Step 5: Line and Cut the Exterior

The first time I made the wallet, the exterior tore and didn't last as long as I would have liked, so for this version I decided to line it. I chose silk for the liner because its resistance to tearing. Basically you just need to cut some silk, and spray-glue it on to whatever you decided to use for the exterior. Go light on the glue.

Once the glue is dry, cut the exterior to 8 1/4" by 3 1/2". That's a good size for US bills. Obviously, you can make it bigger to accommodate bigger bills or receipts. A general rule of thumb is that you want the exterior to be about 1/2" longer than the interior (to accommodate the bills when the wallet folds) plus you want to leave about 1/4" along the edges so you have enough to sew when you fold it over the interior.

Step 6: Prep the Exterior for Sewing

The exterior will fold over the interior 1/4" all around, and the corners are angled, so you'll make those folds ahead of time.

Fold over 1/4" on each edge
Fold over each of the four corners

Also, the exterior is longer than the interior to accommodate the bills, as we talked about in the last step, so there is some preparation to be done for that as well.

Position the interior over the exterior
Cut two 1/4" slits 3/4" apart to form a long tab in the center along the bottom of the wallet
Fold the tab over and sew it down

Step 7: Sew the Exterior to the Interior

Now you have to sew the interior and exterior together. The main trick here is allowing the excess exterior to meet up appropriately with the shorter interior. To do that:

Align the left and bottom edges of the interior into the left and bottom folds of the exterior
Starting from the upper left of the wallet, sew down the left edge, corner, and bottom edge stopping when you get to the tab you sewed in the last step. Sew with the interior of the wallet facing down so that the edgestitch presser foot gives you a nice, even edge on the outside of the wallet
Do the same on the right side. Note that you'll have to buckle the exterior of the wallet a little to nest the right side of the interior into the right fold of the exterior.
Sew the top edge and corners
You'll notice on the corners in the interior, that there is some excess external fabric points on the trim. Cut those off.