Mix and Apply the Grout, and Apply Sealant
After the mortar is dry, mix the tile grout according to the package instructions and apply with a grout float. Use a diagonal motion to apply the grout, working it into the spaces between the tiles. Let the grout dry slightly, then wipe the excess off the tiles with a damp sponge using a diagonal motion.
Let the grout dry completely before sealing. Once the grout sealant is dry, move the furniture back into the room. Let the mortar cure for four weeks before using the radiant-heat system.
Remember to remove the tile spacers before grouting.
The diagonal motion when applying grout works the grout in between the tiles. A straight vertical or horizontal stroke tends to dig the grout back out of the spaces.