Dash cap install help

I am trying to put on a new dash cap. it fits close but doesn't cover down over outside lips. Directions say to just start cutting away material, but before I do this I thought I would ask if anyone else has done this and had success. I do not want to destroy my dash pad unless I know the end result will be nice. not looking for a show piece but don't want crud either. My dash pad has two open tears now so is not just a restain to fix. Just dashes wants $700 to fix it. out of my range.