The easiest way to make a war skirt is to start with a belt, then rivet or sew a bunch of leather scraps to it/around it. Try to achieve a layered effect, with longer pieces behind, and shorter pieces in front. Be sure to weather the leather - hit it with rocks, drag it behind a car, basically beat it up a lot. Use mismatching scraps, make the stitching is large and irregular, etc. Add other materials like hessian or scraps of fur. I’ve attached a couple of pics of different skirmish crew war skirts to show you what I mean, though obviously the Imperial Orc players will have fancier ones to show off
EDIT: One thing to avoid is coloured fabric, as different colours are associated with the various barbarian Orc nations. The red on the middle picture is a modular under-skirt that can be swapped out as needed.