Install underbelly brackets -- tighten slightly. Cover with masking tape on top. Calculate location and mark per instructions. Drill 1/8 inch hole in center of each. Remove brackets and calculate adjustment if needed, by viewing holes in masking tape. Drill holes in bed, mount brackets. Remember the bed holes are not crazy critical, they just need to clear the vertical pins. You can always grind off a bit, but you want them as close as possible.
Mount pins then rails in truck. With winch (or very heavy wench) lower hitch onto rails. Only allow your wife's cat or annoying lapdog under hitch as you lower it.