The curtsy was the basic movement of female reverence from the sixteenth century onward, and variations developed, based on the costume worn or the amount of reverence shown. Basically, the lady slid back on the instep of the right foot with the instep pressing the ground, behind and slightly to the left of the left leg; the instep of the sliding foot took the weight, and the lady gradually sank down sitting on the bent right leg, arms falling to the side, and head lowered. The important point was the crossing under of the sliding right leg, this movement being supported by crossed thighs. The lady then rose with her weight on the left foot since this foot did not move during the entire sequence of the curtsy.
source: History of Etiquette