Start from the outside edge of the paper and measure down 7". Cut toward the center of the paper 5". Now finish cutting the 5x7 mat back to the edge of the paper. From the same outside edge that you started with cut 5"x7" mat going the other direction. Keep going around the outside of the paper cutting 5"x7" then 7"x5" and when you finish there will be a little square of paper left from the center.
Wish I could draw a picture it would be so much easier to explain!