You're now ready to grout the tile. First blend the mix (if you've purchased it dry rather than premixed) according to the instructions on the package. Although it needs to be thick, the grout shouldn't be so thick that it's impossible to force it into the joints. Shoot for a consistency between sour cream and peanut butter. You can mix it with water or with a latex compound for greater durability [source: essortment.com].
Use a foam rubber float to scoop the grout. Then run the float, which you should hold at a 45-degree angle, parallel to the tile joints. After the gaps are filled, go over the surface with the float, maintaining the 45-degree angle to the joints. [source: DIYonline.com].
After the tile has set for 15 to 20 minutes and is firm in the joints, clean the face of the tiles with a damp cloth or sponge. Make sure the sponge or cloth remains clean; rinse it out often. Fill in any holes and make sure the tiles are level. Then smooth the grout to just below the tile level using your finger, a damp sponge or the handle of an old toothbrush. After that, give the grout another 45 minutes to dry. When a haze forms on the tile, polish the surface with the sponge, taking care not to rub the grout out of the joints. If you're having trouble getting the tile clean, allow it to dry a bit longer. Using a clean towel, polish the tiles several hours later [source: DIYonline.com, essortment.com].
Lightly mist the grout over the next three days before applying grout sealer. Misting will prevent the grout from becoming brittle or cracking because it dried too fast. Then apply a grout sealer to protect the grout from water, mildew and stains. After a few years, when the grout ages and becomes harder to clean, simply reapply the sealer.