Cover the area you're going to mark with a fat tipped Sharpie or marker pen. Transcribe the pattern you wish to replicate with a scriber or craft knife. Drill holes at the inside corners to show where you're going to cut to. Using a large drill, remove some of the bulk from the middle then, using a sharp, good quality file, file up your Sharpie line. Finish with Emery paper then Scotchbrite.