Step 3: Prepare card slits and coin pocket
First square the right end. Rule a line and cut (picture 2). Scissors are the way to go! (Inner tube has too much friction to cut nicely with a knife).
Now lay out the coin pocket. Place the coin on the end, and mark a line 5 mm (3/16") further in from the end. Two lines for the card slits are drawn 5 mm (3/16") and 15 mm (9/16") from the coin pocket line. These do not go all the way across! Make them just wider than a credit card or whatever card they will hold (picture 3).
The coin pocket line should also be marked lightly on the other side (outside) of the inner tube (picture 4).
Now cut the card slits. Use the craft knife to start, then continue with scissors. Or fold, and cut with scissors to start. Try for fit, the card should slide through easily. If it's too tight carefully make a little wider (picture 5).