Using a darning needle I ran a cord about the thickness of a shoelace through the pleats. Using a cord of a matching (or as close as you can get) color to the background of the tartan helps to keep it hidden, as does the belt which should be around the area that the cord is through. A couple of little wooden beads keeps the cord from sliping back through. Two notes: Make sure that the cord is long enough to allow the plaid to lay flat when not being worn so that you can still use it as a blanket and such. Also remember to keep the ends on the inside so that they do not show while being worn. That's how I did my two.
Sir Didymous,
Does the needle have an eye? You used it to thread the string through the pleats?

Do you have any or could you take some picture, I would love to see your method.
Thanks