Wash the floor well cleaning all nooks and crannies. Scrub grout lines well with a scrub brush and let the floor dry.
Rub the floor with sandpaper. The object is to take off all the clear coating that makes the tile shiny and leave the color intact. Paint will not stick to shiny tiles. It may take several passes to get all the shiny coating off.
Vacuum the floor. Dampen a rag with water, wipe the floor down and let dry. Use a paper towel to apply acetone to make sure the tiles are free of dust or grime.
Apply the stencil to the tiles, one by one, by taping it on top with masking tape.
Dip the flat end of the stencil brush into the paint, then tap the brush flat on several layers of paper towels so there is very little paint on the brush. Tap the brush on the stencil all around the openings, making the edges darker with more tapping and paint and the inside of the design lighter. Apply more paint as needed. Or, instead of using stencils, hand-paint a motif on the tiles using the same type of paint and an artist's brush. Let dry 24 hours.
Dip a painter's brush in clear polyurethane and give the tiles an even coat. Let dry, and give the floor another coat. Stay off the floor until it is completely dry.
Clean tiles with a mild abrasive cleaner. The object is to get rid of the glossy finish. Sand with sandpaper, and wipe down the surface with a damp rag. Let the wall tiles dry completely.
Stencil or paint as per floor-tile instructions, but use an oil-based paint. Let the paint dry completely.
Apply a thin coat of clear polyurethane with a painter's brush and let dry. Apply another coat and let the tiles completely dry before using the bathroom to shower.