Over the distance of the roof, it's real easy to get out of alignement. Just a small amount near the beginning can lead to a big problem by the end of the run.
I make sure the first panel is dead plumb and aligned exactly where it needs to go. Put a tape on the oposite end of the wall and measure that panel to and bottom. Make extra sure it's right.
Then every other panel, measure it top and bottom from that first one. You can put one screw in to hold it in place, but don't put an more then that until you have it right.
