Jigsaw is the wrong tool. You will never get a decent mitre with one. You can buy a sharp hand saw for less than a tenner. Make up a simple mitre box (even out of skirt offcuts) and use that as a jig to cut your mitre. It'll be far more accurate than a jig saw. or buy one if it fits the size skirt you are using https://www.screwfix.com/p/magnusson-mitre-box/4539v @Deleted member 11267 - beat me by seconds!