Allow the thin-set mortar to set. This usually takes about 12 hours. Don't mix the grout before the mortar is set, because you don't want the grout to dry out! To apply, cover the area thoroughly with grout and make sure you fill all the joints by spreading it across the tiles vertically, horizontally, and diagonally. Then fill any remaining voids with grout.
Then, with a moist sponge, sponge away the excess grout and then wipe clean with a towel. For easier maintenance in the future, think about applying a grout sealer.