Mask off the area you will be painting with tape and cover the rest of the room with drop sheets.
Clean the tile thoroughly with warm soapy water, then rinse it with clean water and dry it completely. Use a small scrub brush to get the grout clean.
Paint a coat of tile primer onto the tile, using long, even vertical strokes with a synthetic paint brush.
Let the primer dry for several hours, depending on the manufacturer's recommendations.
Sand the entire primed area with a fine-grit sandpaper and wipe up the dust with a lint-free rag.
Repeat steps 3 through 5 to apply a second coat of primer.
Apply the gloss paint with a natural-bristle brush, going over the grout once first and then covering the entire area with even, vertical strokes.