Take her measurements just as you would for a man's kilt.

If you call up and order from a large-scale highland outfitter, you'll get a lady's skirt that is machine stitched, likely unlined, and with really small leather straps and buckles.

If you don't mind paying a bit more for it, and want a real quality item, call up an individual kilt maker who can make your skirt for you, and you can get a hand stitched item made to the same quality standards as a man's kilt.

Aye,
Matt