First, take the following measurements.
Waist (where the skirt waist will hang)
Hip (at the widest part)
Hip Depth (how far the widest part falls from the waist)
Skirt length (distance from the waist to the finished hem)
Decide how many gores the skirt will have.(A)
This example will have six gores.
Multiply A by 2
e.g. 6 X 2 = 12 (B)
Divide the waist measurement by B (Wm)
Divide the hip measurement by B (Hm)