To install a pickup truck body panel, loosely attach the fender on the truck, shut the door, adjust the fender to let the door open and close without touching it, measure the correct depth of the gap between the door and the fender, tighten down the cowl and fender in place, and secure in place the core support bolts. A feeler gauge, a ratchet set and extension, a socket set, and a pry bar are handy in the project.
Fix a fender on the truck, making sure not to screw in all the bolts, and loosely bolt the cowl and core support. Be sure that the fender moves freely. Move the fender in place to let the door open and shut freely. Once the fender is in the desired position, fasten the bolt on the cowl with a ratchet and socket. Put a feeler gauge in the gap between the door and the fender, unfasten the cowl bolt, and set the correct depth, adjusting the fender if necessary.
Tighten the bolts on the top and bottom of the cowl. Fasten the fender manually before using the socket set to secure it in place. If the fender's curve doesn't match the other panel's curve, fasten screws on the door jamb, and gently adjust the fender with a pry bar. Push the fender with your hip if you want it to move inside, then tighten it. Unfasten the bolts on the core support to fix the gap between the headlight and the fender, and tightly secure it in place with the top and bottom bolts.