Keep in mind that Steve is a kilt maker, so he notices things most people would miss.

What he means is that, if you look at the surface of a twill fabric, it has a sort of diagonal texture, and that this diagonal texture should run as he's indicated.

And, while the great kilt may have been pleated in that fashion, some of us doubt it. You may find it easier to sew in belt loops on the inside of the fabric, run a cord through it, and tighten the cord up each time you put on the kilt. You can find more details at http://albanach.org/drawstring.htm