Required items to make rangoli on water:
a shallow bowl, or a plate. You could also use a shallow tray. (It can be a steel, plastic or a glass bowl).
a couple of spoons of oil or ghee (clarified butter).
white rice powder, or white distemper powder.
colors of your usual rangoli powder/dye to use for coloring your rangoli.
water to fill up your bowl or your plate.
Procedure to make the rangoli:
Plan on a design to make as your "rangoli on water".
Choose a place (could be a tabletop or an altar) to display your "rangoli on water".
Take your time to decide upon a place for this, because once you're done with this project, you wouldn't be able to move your "rangoli" without disturbing it.
Any shifting to the arrangement of the bowl with the "rangoli on water" once the rangoli is made, could ruin your design.
You could use a glass or a steel bowl, tray or even a 2" to 3" deep plate as a canvas for the rangoli.
Place your bowl/plate in your chosen space, and apply oil or ghee (clarified butter) to the inside surface of the bowl/plate.
Pour some cold water into the bowl, filling it all the way to the edges of the bowl.
Sprinkle fine rice powder or white distemper powder on the surface of the water in the bowl.
Draw your planned rangoli design on the surface of the water and fill in the colors as required.
Stand back and admire!