Porcelain tile is made with pressed clay dust, which is usually topped with a baked-glass glaze to give it that familiar, glossy surface. However, ceramic tile can also be purchased in its unglazed state, which has a more natural, stone-like look to it. Even if you want to keep that look, it's important to seal the tile, to protect it from moisture and dirt. The surface should be sealed after the tile is installed but before it's grouted, so the wet grout doesn't stain the unsealed porcelain. The grout itself then should be sealed as well.
Run a vacuum over the newly installed porcelain tiles, getting all dust and debris off the surface and out of the spaces between the tiles.
Apply a thin coat of tile sealant to the surface of the tiles, using a paint brush. Apply it only to the face of the tiles, not in the spaces between. Try to keep it off of the sides of the tiles.
Let the sealant dry for two or three hours, then apply a second coat in the same manner. Let it dry overnight.
Wipe grout over the tiles, using your grout float, with the long edge of the float pressed to the surface so that the grout is squeezed into the lines between the tiles. Get as much of the excess grout off the surface as possible, scraping it with the side of the float as you go.
Let the grout sit in the lines for five to ten minutes. Wipe down the surface of the tile with a damp sponge to take up the excess grout while smoothing out the lines. Let the grout cure for at least 72 hours, and up to a week.
Apply another coat of sealant with a brush, this time going over the whole surface, including the grout. Let it dry, then apply a second coat.
Ventilate the area when applying your sealant and grout.