Lay it out on a long sheet of paper. You'll be able to visualize it much easier. buckle end of belt of course loops around the buckle bar. Then must extend around the waist to the hook side of the buckle. The distance from the bar on the buckle to the hook might be 2". So if you add up the belt from the bar to the middle hole on the billet, plus the 2" (or what ever the measure from bar to hook is), that will be the waist measurement. Now you can work backwards to make the belt measurement.
So measure the girl's waist over clothes where the belt will be worn, deduct the bar to hook measurement, and you will have the belt length from buckle end (folded over) to the middle hole in the billet. For a growing youngster, I would add a couple inches and make that the new middle hole in the billet.
If you do a search on this forum, you will find multiple threads with instructions.
Tom