First of all start with dropping the tailgates on both sides of the truck. Simply unscrew the tailgates and take them down.
Next, pull out the taillights from back of the truck. Before doing that, unplug the wires and take them out of the taillights from holes and remove them completely. Make sure no wire is attached with the taillights after you are done with their removal.
Remove the gas tank, which is normally attached with the truck bed. Moving the truck bed with gas filer associated with it can cause damage to it. Also, since you will need to attach the gas filler with new flatbed, you should completely remove the old bed.
Unbolt the truck bed from the frame. Make sure you remove bolts in all areas, as lifting of the bed with some screws attached can damage the bed. After unscrewing the bolts, lift the truck bed and push it towards back of the truck. Space your assistants under the truck evenly so that they share the weight of the entire bed.
Now place your assistants under the flatbed to pull it over the truck frame, which you have just vacated by removal of the conventional bed. Pull the flatbed gradually with space assistants evenly placed underneath it and put it on the truck frame.
Screw back the gas filler with the flatbed. You can look for the space for the gas filler, as almost all truck beds have space for attaching the fuel tank with them. You can tighten the screws so that the fuel tank is secure.