I assume from your first post that you are working in tartan? If so, the following are the easiest directions for doing box pleats.

The simplest thing to do is to pin and sew one pleat at a time, just like you do for a trad kilt. So, if you've pinned everything, I'd take it all out before you start - just keep track of where you want the edges of the pleats to be.

Hold the kilt so that the top edge is at the left and the bottom edge at the right (you'll be pleating away from you, starting at the apron edge. Fold and baste the edge of the apron. Locate where the edge of the first pleat will be, lap the folded apron edge along that line, and stitch on the outside by hand using a blind stitch _through only one thickness of fabric_ (this is different than pleating a knife-pleated kilt).

Locate the other edge of the first pleat, and fold it along that line (don't press). Locate the edge of the second pleat, and lap the fold of the first one along that line. Stitch along the edge using a blind stitch. Fold, lap, stitch until you've sewn all the pleats.

Work your way across the back of the kilt. When you're done, you'll have 8 or 9 big loops of fabric on the back of the kilt. Flip the kilt over, front down. Flatten the "loops" into box pleats.