Josie, most grouts are mixed with cold water. It takes very little water, so add small amounts at a time when mixing just enough to do a small job.
You'll want to stir until it is lump-free, and about the consistency of pancake batter. Let is sit for about 10 minutes, then stir again before using.
From what you say, I gather this is a flat, horizontal surface just below the window. If this is the case, I'd suggest sealing over the grout with a silicone caulking, or replacing the grout with silicone caulk, on this window ledge area. Grout will absorb some water when it's on a flat surface; sealing it or replacing with silicone will ensure this area does not hold water. Good Luck Josie!