How to trim or cut rubber gasket or lining on flat glass ?

Method 1: If the gasket is easy enough to pull out, save yourself the time of trimming it and just pull it out and push it back in afterward. If the glass is very tight or the gasket seems brittle, you may have to resort to trimming. If you’re comfortable with it, you can use a fresh blade to freehand cut the gasket. Some people like to use their thumb as a guide for keeping the knife in a straight line. 

Method 2: Instead of cutting in one continuous line, make little points with your knife to create a dotted line along the trim, and then use that as a guide when cutting. 

Method 3: You can use a straight edge like a piece of wood or spare building material, but it has to be even and easy to hold in place, this can be difficult on larger pieces of glass.