US 10.5 needles
1 ball Cascade 220 (Leftovers from another project might be enough, it's very small)

CO 21 sts
Work in stockinette for 50 rows, ending with a knit row
Begin knitting the flap as follows:
Row 1: k4, p13, k4 (This creates a garter stitch border on the outer edge to prevent rolling. If it helps you keep track, place a stitch marker in between the knits and purls)
Row 2: knit
Repeat these 2 rows 5 more times
Begin adding decreases to your knit rows as follows:
Row 1: k4, purl to last 4 sts, k4
Row 2: k4, k2tog, knit to last 6 sts, skpo, k4
(skpo=slip one, knit one, pass slipped stitch over)
Repeat these 2 rows until you are left with 9 sts total
Buttonhole row 1: k3, BO3, k3
Buttonhole row 2: k3, CO1, k3
From here to the end, knit every row, decreasing 2 sts on each edge until you are left with 1 st and then bind off.
Fold your fabric in half so that the first 50 rows you knit make up the body of the wallet, then seam up the sides and leave the the garter-edged flap hanging freely.
Felt your wallet until it shrinks to credit-card size and attatch a button to the spot where the buttonhole falls.