How do I smoke on a propane grill that has a separate smoker attached?
Generally speaking you should soak the wood chips overnight, for at least a few hours, then place them over a flame in a metal container. 

Basically there are 2 types of smoking. "Cold smoking" uses really low heat while keeping the food away from direct flames/heat. With the other, "hot smoking," you're cooking the food while you're smoking it.

You probably want to hot smoke brisket, but you still need to keep the flame low or else the outside will be done long before the inside.

In case it's not obvious, the wood chips need to go into a metal container without water. Wet wood chips smoke better then dry, but you don't want to boil the wood chips.