BLUF: it depends on what temp you want.
"For 225-250° cooking:
Assemble the cooker, add cool water to the water pan, and put the lid in place.
Close all three bottom vents. Leave the top vent fully open for ventilation throughout the entire cooking process.
Keep an eye on the thermometer. The initial temperature may be 400°F or higher depending on the type and amount of fuel you're using, but within a short time it will begin to drop.
If you're loading up the cooker with a lot of meat, add the meat and smoke wood when the temp comes down to about 350°; if cooking smaller quantities, wait until about 300°F. The temperature will drop quickly into the 225-250°F range once the meat goes on.
For 325-350° cooking:
Assemble the cooker, put the empty water pan in place, and put the lid in place. Line the water pan with wide aluminum foil for easy cleanup.
Adjust all three bottoms vents to 50% open. Leave the top vent fully open for ventilation throughout the entire cooking process.
Add the meat and smoke wood to the cooker immediately. The temperature will drop quickly into the 325-350°F range once the meat goes on."
Here's article:
http://www.virtualweberbullet.com/fireup1.html