It depends on who is in charge. If it's a state of CA fire, the state has the air attack direct the lines (usually by description), the Feds will use leadplanes to lead the air tankers in. Here's an example of following the lead in.
If it is initial attack on the fire, the tanker is given a rough GPS location and then the pilot decides best attack on the fire. Fixed wing tankers aren't usually used for dropping on the fire, but laying down lines tied in with natural fire breaks, to prevent the fire from spreading. Helicoptors with buckets of water attack the fire directly (usually).