You CAN use some of the on-line ones, BUT for acceptable results . . . BARB'S BOOK is the BEST and produces a SIGNIFICANTLY better kilt in every form or fashion.
My first kilt I made, I used online instructions (found here while looking for them and joined a couple months latter). I then bought Barb's book and IT IS SOOOOOOO MUCH BETTER that the cost is WELL worth it.

My bookmarked instructions:
http://users.telenet.be/jbruyndonckx/kilt_making.html
http://www.scottishdance.net/highland/MakingKilt.html
http://users.tinyonline.co.uk/chegc/kiltsite/kilt.htm

The first one isn't really THAT bad, but the ones made AFTER getting Barb's book, even if they are box pleats, are SO MUCH better that the first one looks crappy to me now.