I found these instructions on DIY:
"First, wash the surface to with a 50/50 solution of household ammonia and water to remove soap films from bathroom surfaces or grease from kitchen walls. I also recommend you wash with a mild acid solution which is used to clean ceramic tile of excessive grout. This can be found in the tile section of most home centers,Ace hardware and tile distributors.Rinse well with plain water, and let the surface dry thoroughly.
Next, prime it with a high-quality adhesion-promoting primer like Zinsser's latex-based Bulls Eye 1-2-3 Primer-Sealer. Bulls Eye 1-2-3 adheres tenaciously to ceramic tile, glass, and other hard-to-stick-to materials. The primer provides an "anchor" for the topcoat so it can't be rubbed, chipped or scraped off easily.
If the area is going to be subjected to a lot of water such as the tile in the tub area, use BIN an alcohol base primer for maximum durability.
Follow the primer with one or two coats of a high-quality finish paint. In moisture-prone areas alkyd (oil base)finishes are preferred, as they provide a harder finish than water-base paints and are more washable and durable. There are also several epoxy coatings that are excellent for durability, check with your local paint store for availability.
If you want to paint the grout a contrasting color, you can go back over it with an artist's brush.
Your finished project will be fully cured in appx 2-3 weeks and you should avoid harsh abrasive cleaning agents, soft scrub is a good general cleaner for painted surfaces.
Zinsser's Bulls Eye 1-2-3 Primer-Sealer can be found paint stores, hardware stores, and home centers across the country. For more information, visit Zinsser's web site."
Hope this helps.