A heavier piece of vinyl with a turnbuckle was made to determine the curvature of the walls. Others have reported a difference from window to window and even side to side but I found that, with my coach, they were all the same within acceptable tolerances. This curvature was transferred to the square vinyl. Remember to add spacing for the width of the blind header at the top and the width of the lower part of the blind PLUS the thickness of the window trim. I found that scuffing the surface of the vinyl with an orbital sander and marking with a #2 pencil worked best to mark the cut lines.