Kind of a small picture, but just tie the handle bars to the corner rings in the bed of the truck. Make sure the bike is secure, and it's okay if you have to angle the bike a bit to close the tail gate.
If you can, get soft straps (
http://www.amazon.com/Soft-Loops-Tie.../dp/B002MYLB1Y) or some way to keep the hooks off of your handle bars. Loop the soft straps around the lowest bend of your bars just outside of the triple tree, and hook the tie down straps into the loops. Then run the tie down straps to the eyes in the corner of your bed. That's it. It should be self evident once you get the bike in there.