I have found that when working with the stiffer fabrics used in Modern Kilts that making the kilt a cylinder does not always work.

For your Traditional Kilt follow Barb's instructions and make the Kilt a Cylinder.

For your Modern Kilt take the waist measurement as usual. When taking the hip measurement though, pull the tape measure out from the front of the hips and simulate that the apron is falling straight down from the belly.
This will give you a larger measurement for the hips and allow you to put some taper into the back of the kilt.
This method is only used on the stiff fabrics of modern kilt because they do not form and mold to the body as well as Wool.