1. Remove old grout.
Use a grout saw or utility knife to scrape hardened grout out of the joints. Clean tiles with a nylon pad and vacuum out the dust.
2. Trowel on new grout.
Mix a batch of grout, according to the directions. Use a rubber trowel. Apply in a circular motion. Hold trowel at a 45 - degree angle to remove excess grout.
3. Remove excess grout.
Use a small amount of water and a sponge, wipe down the tiles. You may have to repeat this process 2 or 3 times.
4. Buff the surface.
Your tiles will be left covered with a fine haze. Once the grout is dry, you can remove the haze, by wiping the tile surface with a soft cloth.
5. Seal the grout.
Don't forget to seal the grout.