Just put the kilt on, wrapping the underapron around you _without_ putting the strap through the buttonhole. Wrap the apron around you as if you were going to buckle the kilt, and measure how much you need to move the apron edge buckles. Then remember that you must move the _underapron strap_ an equal amount toward the center of the underapron. If you don't move it, you won't be able to buckle the underapron tightly (and moving the buckle next to the buttonhole won't do anything).