Lay credit card/ gift card on the wallet, and fold it to make sure it fits. If it does, continue to the next step.

If you left it there, cut around it. If not, then cut wherever you feel comfortable. Then make the credit card fits. (don't worry, your wallets not broken!!)

Hold the front part, and tape the back parts back together.

Once again, make sure the credit card fits, and fold it to make sure.

I did 2, but you can do as many as you like.