Renewing old grout has always involved long hours of hacking away at old, worn grout with a tiny grout saw. Simplify the job by purchasing a grout removal kit that attaches to a Dremel tool. (See photo below). It has a high-speed carbide bit that effortlessly chews away old grout, and guides that keep you from chipping the tile edges. Make sure to wear safety glasses while grinding (Photo 1). Use the grout saw to scrape out edges and corners and to clean out the joints.
Vacuum up any dust or debris left after the grinding process. Take a chunk of your current grout to a home center or tile shop to find a match. Also buy a latex additive to mix into the new grout to make it more durable. Mix the grout slightly thicker than peanut butter and then apply it (Photo 2).
Let the grout set up for approximately 20 minutes until a film develops over the tile, then clean the area as shown in Photos 3 and 4. After the grout has dried for a week, protect it with a grout sealer.