Cut a piece of watercolor paper to fit your tray.
Cut out some basic flower shapes on the tissue paper.
Wet the base, then scrunch up the tissue paper.
Set it around the base paper and wet it more with water and a paintbrush.
Remove it.
Place a leaf on top and paint over it with water.
Continue to add cutouts of the tissue paper, setting them around in a design you like and brushing the tops with a wet paintbrush.
Let the painting dry.
Add a touch of black marker to outline the flowers and leaves if desired.
Glue the painting to the tray.
Add a piece of plexiglass to the top and use clear caulk around the edges to protect your art.