Paper, white T-shirt or plain apron
Take the stems off the flowers and lay them out on whatever you are hammering them on. Place a paper towel under the top layer of the T-shirt so the color won't soak through to the back.
Put a piece of plastic wrap on top of the flowers. Hammer the flowers until they are flat. Remove the plastic wrap off and remove the petals that didn't come off.
If you did a T-shirt or apron, soak it in salt water to make it permanent.