Real simple. Just did this last May. Ride the bike straight into the bed. Put the front tire up against the front of the bed. Put it in gear and hit the kill switch. Let the clutch out. You can even drop the clutch and stall it in that position if you really want to be anal about it. I just didn't want to start my relationship with the FZ6 on a sour note.
Hopefully you'll have a friend along. Stay seated on the bike to keep some compression in the suspension. Have your friend secure a ratchet tie-down to each of fork, right at the lower part of the triple tree. Don't ratchet them much at all, because you're already compressing the suspension. It's better to use your weight and not ratchet much than it is to just ratchet until it seems tight enough. It's too easy to overdo it and ruin your fork seals. Trust me, I did this years ago with an old bike.
Then secure the rear via the passenger pegs mount. The rear isn't too important. The front tie downs keep the bike pressed firmly against the bed and won't allow the wheel to turn. All the rear has to do is help keep the back of the bike straight. You don't even need to tie down the rear, but I would do it just in case you have to brake hard or make an emergency maneuver while transporting it.
You'll have to leave the tailgate down, but don't worry. I drove mine 100 miles on bumpy PA highways and the interstate. It was rock solid. I didn't even stop to check on it as I had planned.
Also, talk to whomever you're getting the bike from ahead of time. Odds are, they'll know exactly what to do. See if there's an incline nearby and back the truck up to it so it's less of a climb up the ramp. Enjoy your bike, and be safe my friend!