We put a few pieces of used paper under the cloth, on the cement, to prevent the fabric from tearing. Then Maddie just started pounding on the petals with her smooth stone! Younger children can lay out their flowers, cover them with paper, then smash the paper with the stone, pressing the colors onto the fabric beneath. Doing it this way will prevent smashed fingers while they're trying to hold the petals in place.
Some petals work better than others, and some change colors after they dry. All will fade over time. Even under glass, natural light will eventually fade the colors.