Pretty easy actually.

Take a 200' rope and hold it in one spot in a field. Have your buddy walk out until the rope is tight and then as you turn he walks around to form a perfect circle.

Then on the center spot of that line do the same to form several circles on the boundary the earlier formed line.

Really you don't even need another person...I stick will do.

So one person, one rope, one stick and an afternoon to kill and you have a crop circle.