Gather enough fresh flower petals to loosely pack one cup. Peonies and poppies are commonly chosen for their photosensitivity.
Place the flower petals into a mortar. Grind the pedals into a paste with a pestle as you gradually add 2 tbsp. of distilled water or isopropyl alcohol.
Line a bowl with two layers of cheesecloth. Pour the flower petal paste into the lined bowl. Gather the corners of the cheesecloth and squeeze until all the remaining liquid drips into the bowl.
Paint one side of a piece of white matte card stock paper with a thin layer from the liquid in the bowl. Dry the painted card stock overnight in a dark room. A hair dryer on the warm air setting can also be used from a distance of 6 inches or more to reduce the dry time to just a few minutes.
Cover a flat surface in a sunny location with a page from a newspaper. Place the card stock in the center of the newspaper with the painted side facing up.
Place one or more flowers onto the card stock. The form of the flower creates shadows that protect the photosensitive paper from sunlight. Cover the flowers with a sheet of clean clear glass.
Remove the glass and flowers after a minimum of three days in direct sunlight. Shadow flowers can be left to develop for week or longer for more distinct images.