Step 1: Cut the Paper
You'll need 9 pieces of paper. Two large ones for the front and back of the case, four for the sides, top and bottom, 1 for the front of the flap, and 2 triangular pieces for the sides of the flap.
The attached template could be printed on heavy paper and cut out, but where would the fun be in that?
Step 2: Coat With Tape...Lots of Tape
You'll want to cut strips of tape 2" longer than the paper...I chose to use vertical strips. The pictures show this step better than I can put it into words. after covering one side of one of the large rectangles, place one of the skinny side rectangles next to it, with about an 1/8" gap. This creates hinge to allow the pieces to fold when it comes time to put it together.
Step 3: Form the Pocket
Now, fold the outer side piece over so that it touches the adjacent face plate. This will make the interior part where the cards will be kept. Press down the tape connecting the two pieces to make sure you have a good seal.