Dwight's method is spot on. He forgot to mention, you will probably build a few holsters before you get it just the way you want. They will certainly be functional, but perhaps not just how you want them to until you get your process down. I have several of those some I use, some I don't.
I started using some 1/8 inch foam sheet (Amazon.com: MEARCOOH Black eva Foam roll, (1mm to 20mm) Premium Cosplay EVA Foam Sheet,2mm Thick,49"x13.5",High Density 86kg/m3 for Cosplay Costume, Crafts, DIY Projects : Arts, Crafts & Sewing) to model new ideas or patterns, which gives me a better idea how things will work. It's a lot less expensive than screwing up your leather.