I sort of freehanded this a while back, having figured out the formula when I didn't manage to get a hold of the tutorial again... It's the accordion style section that's the trick. I'll try to explain while we wait for the tutorial to show up.
The width should be the same as the long side of a credit card plus about an inch (that's including seam allowance, in my case). The length is something like this, also including seam allowance, in my case:
2''+2''+2.5''+2''+2.5''+2''+2.5''+3.5''
where you fold on each + sign. The first fold makes the wrong sides face each others, the second makes right sides face eachothers, if you know what I mean... the first 2'' is the face of the first pocket, then it folds down 2'' to create the inside along with the following 2.5''... the extra 0.5 gives the stairway effect. Folded up, the bottom of the first 2'' and the "top" of the last 3.5'' should meet up along the bottom edge.
To increase your wallet's life, put some thin but stable interfacing on the first 2'' segment, because the opening and closing will be wearing your fabric down a bit if it doesn't have any support.
Top stitch along the
top of each pocket to give it some stability.
Now put whatever closure part you want to put on the front pocket before sewing anything more together, because you probably don't want it poking through to your cards.. so attach that on the face of your first 2'' bit. (through the interfacing, but not the second 2'' segment, just the first one!)
Then you sandwich your folded up accordion between two pieces of fabric that are the same width and have a length that's 7''. (really it's 3.5'' plus however long you need your flap to be, but it's easier to just make it twice as long as the height of the pockets, then you can trim it afterwards.) You'll want to put interfacing on one of these pieces as well - if you put it on both, you may have difficulties turning it later, so I just put interfacing on the outside fabric if I use a snap button that goes straight through.
Put your pocket accordion with the back (the 3.5'' side) on the face of the fabric that you want to be the inside of your flap, raw edges lining up along the bottom, and then put your outside fabric on top of this, right side facing the pockets as well. Sew along the long sides and the bottom, leaving the top open for turning. ...and turn!
Now you just need to attach a closure to the top of the flap, lining up with your button on the pockets (make sure it'll close tight over the pocket accordion, but not so tight that you're not able to close it once your cards are in) and then sew the open edge closed, trimming it to the length you want it to be.
And yes, my way of doing this means that the cards in the frontmost pocket will be sticking up a little more than their brethren, because the first pocket will be sacrificed to the gods of seam allowance. You can see that effect on this piece.
Just add seam allowance to the first 2'' and the last 3.5'' (and the length of the back and front pieces) if that bothers you.
...did that make sense?
