Grout is mortar that's used to bridge the gap between tiles and help set them in place. Polyblend grout is specificially formulated for interior tiling, with added chemicals for more stain resistance and colorfastness than regular mortar. It comes in various colors, so consider the color of your tile when choosing your grout. Grout lines between the tiles should generally be between 1/8- and 1/2-inch thick. Grout can stain unsealed tiles during the grouting process, so make sure your tiles are presealed.
Mixing the Grout
Mix your grout in a bucket, using a medium-sized putty knife to stir it together with water. Your goal is to get the grout to the consistency of thick mud or cake frosting. (The amount of water needed will vary with different brands of grout.) Put an inch of water in the bottom of the bucket, add enough grout to cover it, and begin stirring. Add water and grout alternately until you've got it to the right consistency. Don't make more than a gallon bucketful at a time. Let it sit in the bucket for 10 minutes before grouting. This process, called "slaking," ensures that the chemicals in the grout are properly melded.
Applying the Grout
Grout is applied by scooping it out of the bucket with a rubber grout trowel and spreading it over the tiled surface so that it's pressed down into the lines between the tiles. After initially spreading it, squeeze it off the tile face with the edge of the trowel, running the edge perpendicular to the lines. Let it sit in the lines for a minute, then use a well-wrung sponge to wipe down the surface. Use the widest part of the sponge and wipe gently so you're taking up the excess grout on the tile face but not digging it out of the lines. Make sure the lines are smooth and even when you're done, and let it dry.
Grout is naturally absorbent, which isn't what you want on a tiled surface. Solve this problem with grout sealer. After the grout dries and sets for two days, apply the clear liquid sealer over the grout lines, using a small paintbrush or a sponge-bottle applicator. Keep the sealer only on the grout lines and not on the tile itself. If the lines are narrow and you're getting the sealer on the tiles, follow up immediately with the wetted corner of a cloth to wipe it off the tiles. At least two coats of sealer should be used in high-moisture areas like bathrooms.