From the Weber blog:
First, get your grill ready:
Gas Grills
Preheat your gas grill with all burners on high for about 15 minutes. Then moderate the heat by turning off one or two of the burners: if you have a two-burner grill, turn off one burner. If you have a three-burner grill, turn off the center burner. If you have a four-burner grill, turn off the two inside burners. Wait until the temperature reaches about 325°F.
Charcoal Grills
Arrange hot coals evenly on either side of the charcoal grate until the internal grill temperature reaches about 325°F. You can add or remove charcoal to help reach the correct temperature, but keep the number of briquettes fairly even on each side. Place the cooking grate over the coals.