You could take the time to make some templates or just measure and cut out the 4 pieces. Dimensions are found on the template picture. The pen holder is sized for standard Bic/Paper Mate style pens (or mechanical pencils). If you use pens w/ a larger diameter, adjust the overall length of the pen/card holder piece when you cut it out.

I use a utility knife and straight edge to cut out the leather. I buy the 100 pack of blades and use a fresh one whenever they begin to dull.

Round the corners of the main cover piece and the outside corners of the front and back flap. I use a 3/8" washer...and you can use whatever size you want...or just leave the edges square.

Make the slant cut and notch on the business card holder. For the slant I make a mark on the top edge 2 5/8" in from the right edge and down 1" from the right edge. The notch is 2 3/4" in from the right edge. The notch helps when stitching on the business card holder because this piece isn't laid flat, it's wider so it can be bowed out to fit more cards. I used an English point strap end punch to cut out the notch.

Mark the edges of the back flap where the glue needs to go. Mark the center lines on the front flap and biz card holder, mark the glue lines as well. First glue line is 7/16" wide from the left edge, second glue line is 5/16" wide and is 50mm in from the left hand edge.

Skive the underside of the left edge of the biz card holder (where the 7/16" glue line is). This edge is glued to the underside of the front flap.

Apply contact cement (Barge) to all surfaces, wait for it to tack up / dry and glue back flap to cover, and the edge of the biz card holder to the front flap. Tap glued edges with mallet.