Put the piece of card inside the bag to prevent paint going through to the other side
Paint on your design
Wait for the fabric paint to dry - overnight, or at least a few hours. Depending on your paint instructions
Don't forget to take it shopping with you.
Recipe Notes
The reusable bags should be washable, although leave it a bit longer to do so just in case. I only did one side of each bag but once the side is dry you could turn over and repeat the pattern on the reverse as well.