Find a flat and solid piece of ground on which to set up the ladder. If there is not flat or solid ground, put down a flat piece of strong plywood to solidly support the ladder.
Place the feet of the ladder in the center of the flat area or plywood and lean the ladder against the building. The ladder should lean at an angle that creates a four-to-one ratio between the height and the distance of the feet from the building. If your ladder is 16 feet high, its base should be four feet away from the building.
Climb the ladder and drive a nail into the building on the outside of each leg of the ladder to keep it from sliding to the left or the right.
Have someone else hold the ladder to steady it if you have any doubt about its safety.