How to keep a Z-Notch Lined up

To all you seasoned truck builders. Im about to Z-notch the frame on my Rodeo. The question I have is, How do you keep the frame straight? How would you set up the frame and cut it, and then what would you do to make sure that you are welding it back together straight. I dont want to end up with the back of my truck drivin one way while the front points a different direction.