To help the natural dyes take to the fabric, you need to prepare the bag first. Wash the bag to get rid of any residue.
Dissolve 1/4 cup of alum in two cups of hot water. Soak the bag in the mixture, wring it out, and let it air dry. Iron to get out the wrinkles.
Slip a piece of parchment paper inside to keep the dye from leaking through.
Gather flowers and leaves. If the flower is really full, with lots of layers of petals, you'll want to pluck a bunch off first, so the prints come out nicer. Lay your flowers out on the bag, and secure them with lots of masking tape.
Put the bag on top of your board and hammer away. Make sure you hit every part of the flowers.
Peel the flowers and tape off to reveal your print. Add some additional flowers in random areas and hammer as needed.
When you are done, let everything dry, then put another piece of parchment paper on top of the bag and iron over your design with a hot, dry iron on the highest setting. This will help the colors set in and keep them from fading.