On top of the fuel tanks, top and center sections of the frame, brake lines, firewall, fender wells, inside all the body panels on the bed, under the doors and lower door jambs, under the rockers, floor, transmission hump, behind the bumper etc. Any major surface under the truck that can be prone to rust. If you have a spare tire under the truck, remove it to get the the underside of the rear tank.
If you are using oil undercoat, avoid spraying any rubber parts like you're weather strip, motor mounts or body bushings. The oil swells up the rubber and they crack when they dry up.
Also void spraying the muffler and catalytic converters, they will smoke and stink for a good week.
Before you start, give you're under body a real good cleaning with a hose or pressure washer to get all the loose stuff off. When its dry, you will have a clean surface for the undercoat to stick on.