1. Scrape out the old grout completely, using an awl, a utility knife, or a grout saw (or all three). Brush out the joints with a grout brush. Mix the grout according to the manufacturer's directions, but mix only enough for one section, as the grout dries quickly.

2. Spread the grout liberally over the tiles, using a rubber grout float. Then, work the grout well into joints by holding the float face at a 60° angle to the tile and dragging it forcefully over the tiles at a 45° angle to the joints. Let the grout set for 10-15 minutes, then wipe away excess with a damp sponge, rinsing the sponge frequently.

3. Let the grout dry for about 1 hour, then wipe away the powdery residue from the tile faces with a soft, dry cloth. Apply caulk around a bathtub or shower stall. Do not use the tub or shower for 24 hours. Seal the grout after it has fully cured (check manufacturer's instructions).