I'm about to do this myself so I'll add some questions:
Where do you start? Most things I've read say start in the center.
What is an easy way to measure your bottom chord solo?
What length of screws if you're just supporting the panels?
What's the dimension of the ceiling? Many panels are 9" centers for the major ribs. Unless you're super picky and a little different reveal from a major rib from one side to the other would bother you, I'd start on the side. You could measure the distance between the walls (in inches) and divide by 9" to figure how to start so that it's even, or closer to it.

You can use 1" screws but those short bastards are not the easiest things to hold on to and drive in. 1 1/2" is a lot more friendly.
Plan out your seams and order accordingly. Seams facing away from your preferred perspective will help with the appearance.
Personally I'd rather see one or two nice straight lines (seams) over randomly (and wasteful) seems.
Measuring overhead by yourself, set a roofing nail and there's a slot on the end of your tape measure that'll accept it.