Store grout inside, away from heat. Do not store in direct sunlight. If outside, cover grout with tarp or place in shade. When mixing grout, use cool, clean water only. Moisten tile, especially edges, with a damp sponge.
More absorbtive tiles require more wetting.
This prevents the rapid loss of water from the grout into the tile. Use shades or screens to block sunlight or wind. Besides preventing evaporation this also allows the grout to cure uniformly in color. If possible, schedule grouting to be done during the coolest part of the day. Covering of the installation should be done immediately.