Tile and grout may look solid and impenetrable, but in fact both materials are porous and vulnerable to moisture and stains until they're properly sealed with a liquid sealer. Many ceramic and porcelain tiles come pre-sealed and glazed, so all you have to do is install them, grout them, then seal the grout lines. But if you're dealing with natural stone or other unsealed tile, then you have to make sure to seal the top before you grout, so the grout doesn't stain the tile. Then re-seal the whole thing after the grout is in.

1
Lay out your tile using your tape measure, pencil and level, with intersecting lines through the middle of the area to be tiled. Use your notched trowel to spread down thinset mortar in one corner of the intersection, for a few square feet.

2
Press your first tile into place, using the lines as guides. Press additional tiles into place alongside the first, putting tile spacers between them. Continue until the whole area is tiled, cutting the tiles at the end as necessary with a tile cutter. Remove the spacers and let the tiles set overnight.

3
With your brush, apply tile sealer to the face of the tiles. Apply it so it thoroughly covers the face of tiles, but make sure it doesn't get into the spaces between the tiles or on the sides of the tiles. (Sealer in the lines or on the sides could prevent the grout from properly setting.) Allow the first coat of sealer to dry to the touch, then apply a second coat in the same manner.

4
Scoop up a large glob of your pre-mixed grout with your grout float. Apply it directly to the lines between the tiles, pressing it into the lines and using the edge of the float to squeeze it off the tile face. Work in sections of a few square feet, letting the grout sit in the lines for a minute and then wiping it down with a damp sponge. Continue until the whole area is grouted.

5
Wipe the area with a damp sponge again in about 30 minutes to take up the residual grout film that will form. Repeat 30 minutes later, and again after that, until the film no longer forms.

6
Let the grout cure from a few days to more than a week, according to the instructions on the packaging. Re-apply your tile sealer with a brush, this time covering not just the tile face but the grout lines as well. Use at least two coats.