I make kilt liners and I pin them into my various kilts with the smallest size safety pins onto the tape which is included at the inside of the waistband.

The waist band is just a narrow strip binding the top of the pleats, not adding anything to the length of the kilt. I sew it to the outside of the kilt, then fold it over to hide the raw edge and then pin it over the tape which is laid on the inside of the kilt and sew it down.

Basically it is a piece of fabric slightly shorter than the kilt with darts to shape it into the waist and long enough to stretch all the way around the inside of the kilt from the edge of the apron to the edge of the under apron - so when worn if the apron flies there are still three more layers of fabric between me and the public, and if the pleats do a flutter there is still a decent almost knee length covering.

I make a small hem, and I would suggest rounding off the corners of the free edges so as to make it continuous on all three free sides.

They are easy enough to make once you know the position length and width of the darts required for the shaping.

If you don't have safety pins it is easy to tack it in place temporarily then remove it for washing.

It should be tested carefully for length, too long and, as it will be closer than the kilt with the weight of the pleats to hold it might result in it catching in the bent knee when stepping over anything - out in rough country on a wet day it will strip the skin off the back of your knees in minutes.