Regrouting a shower

Remove old grout: Use a grout saw or utility knife to remove at least 2 mm of old grout.
Mix grout: Follow the manufacturer’s instructions to mix the grout with water until it has a thick paste consistency. Let the mixture sit for up to 10 minutes.
Apply grout: Hold a grout trowel at a 45-degree angle and spread the grout across the tiles in multiple directions. Apply extra pressure at joints to ensure even spreading. Regrout the tiles at least three times to fill all lines.
Fill joints: Use a rubber grout float to spread grout into the joints at a 60-degree angle, working in alternating directions.
Wipe away residue: Wipe away any residue.
Seal the grout: Seal the grout.