Old grout should be removed and repaired sooner for preserving the tile.
1. The old grout should be removed using a grout saw running the saw back and forth over the grout line to break up all of the old grout.
2. The tile joints should be vacuumed out to remove all of the dust and broken-up grout. The joint should be wiped with a dry towel to remove the remaining grout.
3. The grout should be mixed in a small bucket according to the instructions on the packet.
4. A plastic putty knife should be used to scoop out and push the grout down into the grout joint.
5. A rubber grout float should be run back and forth over the joint and pressure should be applied to make sure that the grout gets packed into the joints.
6. A damp sponge should be used to wipe out excess grout and the grout should be allowed to dry.