Place your fabric on your printing board. Put some paint on a plastic plate and apply the paint with a foam roller to the textured side of the tray. Place the tray on the fabric. Don't press too hard, you only want the raised parts of the tray printed.
Here are some examples of how the fabric looked after the printing:
The styrofoam trays of pizza's can also be used as stamps. With an empty ballpoint I pressed circles into a pizza tray. By pressing the styrofoam compresses a bit and creates a slight heigt difference. The tray is thin, so I needed something like a handle to lift it from the fabric. For this I folded a piece of cellotape partly double and pressed this to the back of the tray.
I placed my fabric on the printing board and applied the paint with a foam roller. I pressed the tray onto the fabric and repeated this. This is the result:
