Cut the mat board with your mat cutter to the dimensions of the frame so that it fits securely inside the oil painting frame. Keep the sides of the mat at two inches.
Measure the dimensions you need to leave a box in the middle of the mat board with a ruler to display the oil painting. Place markings on the back of the mat board to know where to cut.
Cut the mat board with the mat cutter at the locations of your markings. Use the mat cutter's blade to ensure all corners are evenly cut.