If I am understanding correctly, this is the phillabeg from Celtic Croft. If so, this is the untailored garment that you pleat each time you wear it and belt it on.

In this case, just pull up the kilt a little higher on the waist. Or, if you really don't want it any higher on your waist, just pull a little more of the material over the belt. You don't need to do ANY sewing.