I normally use the back of an old rejected print to make a mask. Aurore is right about "too obvious marks", but I avoid this by laying the old sheet face down on a pile of boxes of photo paper, so that the projected image is smaller than the new print I want to make. I then draw around the edges of what I want to dodge or burn, and cut out the mask. All I need to do then is estimate how high I need to hold (and move) my mask above the new print.