Watch this videos on how to fold origami fish. I sourced several different videos to learn how to fold: #1, #2, #3. You can also use an origami book for reference.
Fold your fish according to the directions in the videos or book. You may need to cut the origami paper into rectangles to mimic dollar bills.
Download and print out your cards. Cut the paper in fourths.
Adhere the origami fish however you like. You can glue them onto the cards with glue stick or tie string around a paperclip.
Clip the paperclip to the fish. Cut a slit in the card to wrap the string through or tape to the backside.
Be sure to sign the cards and/or write a message on the back.
Notes
You can also fold dollar bills into origami fish and attach those to your cards.