" Rounded " edge goes toward window frame. When I did mine, laid out glass on a piece of 3/4" plywood, fit rubber around the glass, drove nails around the outside perimeter to hold the shape and cut to length. Rubbed a light coat of grease on the face of the rubber and set potmetal frame on top - hole locations show on the grease. Drilled and countersunk for the holes. Siliconed the glass into the gasket, set the frame on top, and had a friend hold in place while I screwed from inside.
