Dip a sponge in clean water, let the excess water drip away, then lightly moisten the grout to soften the efflorescence. Soaking the grout with water is not necessary.
Scrub the grout with a stiff nylon brush or a nylon scrub pad to try and remove any light/mild layer of film on the grout.
Apply a haze remover and scrub some more if the haze is too much to remove with the water alone. Make sure you get a compound that will work with efflorescence, and read all application instructions.
Apply a strong, phosphoric acid-based grout cleaner if you still have problems with the haze. This should only be used in extreme cases of efflorescence that have been present on the grout for at least three weeks.
Rinse off all of the grout with another application of clean water and the sponge. The amount of water shouldn't matter here since the purpose is to remove all of the cleaning solution(s).