I find box pleats a little mystifying, but basically the width of sett of the tartan is the width of each panel and a sett is folded under on each side of each panel so from hem to hip there are three layers of fabric, and the inner folds meet behind the centre of each panel

You can have either an odd or even number of panels - depending on how many you need to go just over half way around.

At each side I would have a large pleat under the apron of one whole sett, then I assume you create a flare at the inner edges of the aprons and on the outer edge of the upper apron, as for a traditional knite pleated kilt.

What puzzles me is creating the narrowing to the waist - is the extra material cut out or just overlapped at the back of the centre of the panels so there are strips where there are 5 layers of fabric, or are the panels made so they do not meet at the edges at the hips and accomodate the extra width that way.

I have started out to make a box pleated kilt and chickened out unable to achieve the required shaping in a way I was happy with. I shall have to dig it out and have another think about it.