For below AND above knee covers...
With a ruler or measuring tape to measure the height of the following areas.
Front / Back / Outside / Inside.
Second: Measure the circumference of the socket, about 1/3 down from the top edge. (see video on left)
For the Below Knee full leg covers...
We need only two measurements.
1. The height from bottom of your socket to the thigh (just above your sleeve) and;
2. The circumference of your thigh