What Franco said. Lay your tent on the footprint material and trace it. Cut inside the lines. You don't want the footprint outside of the perimeter to collect water. Tyvek is the easiest to work with and probably the longest lasting. There's sites that'll sell it by the foot - http://www.antigravitygear.com/shop/shelters-accessories/tyvek-ground-cloth-by-the-foot/