The upper leg circumference is the measurement around your leg, near the top of your leg.
You should measure this a bit below your crotch, refer to the illustration for guidance.
Make sure to keep the tape measure horizontal to the floor as you measure the circumference of your leg.
This measurements is also used as a vertical reference to determine the vertical space in which to draft the seat/crotch of trousers. As such, the vertical location of this measurement is important.