Wash the fabric to be used with laundry detergent.
Lay your desired fabric outdoors on a clean, flat surface.
Cut fabric to the size needed. A standard outdoor rug measures 8 feet by 10 feet.
Coat one side of the fabric with linseed or castor oil using the paintbrush. Alternatively, a commercial varnish can be used, but will not necessarily result in a natural product.
Allow the oil or varnish to dry for one day. The oil should soak into the fabric and be firm to the touch.
Apply two more coats, allowing each to dry in the same manner.
After drying the final coat, turn the fabric over and apply three coats to the other side in the same manner.
Lay the oilcloth on the ground in the place of your choice. Be aware that the oilcloth will kill any plants and grass underneath.