I make a pattern first from posterboard, then from 1/8" masonite. I do all the fine sanding and fit the masonite exactly to the opening just as I want the glass to fit. Don't forget to fit the seals and weatherstripping you will be using on the glass to the masonite patteren so that the final cut glass is the correct size when it is cut. And when you pick up your new glass, make sure it matches your masonite patterens exactly.... A bit of extra time spent on the patterns will make the final install on the glass an easy job.