1. Rototill or dig the soil; rake it smooth. Lay the 2-by-4s about 7 feet apart to serve as temporary guides. Place a stake in the soil to mark the patio's center; tie a 3 ½-foot-long piece of string to it, then tie the string's free end to the second stake. With the free stake, trace the patio's outline in the soil, pulling the string taut as you walk a wide circle around the center stake.
Remove the stakes and mark the circle with gypsum. Remove the 2-by-4s from the sides; then place the carpenter's level on one of them to make sure soil is level. (Recheck level at every stage.)