How do you hook up tractor to semi trailor?

You make sure the kingpin lock is pulled out from the fifth wheel, then you back up to the trailer - keeping the truck centered with the trailer. Depending on how the trailer was dropped, you may need to deflate the tractor air bags to lower your fifth wheel height. When you back under the trailer, you'll hear the kingpin lock engage. Pull forward, to ensure it's locked in place. Once that's done, you'll reinflate your cab air bags, get out, visually check the fifth wheel and kingpin lock, then you'll hook up your pigtail and air lines.