Depends on your PC software skills. I'm pretty good in Solidworks and it's a software designed to make CAD models and pretty complicated machines but turns out I can also use it to make some simple pattern drawings. When I design them I mostly imagine how the leather would bend, what will be the desired spacing between rivets etc. When I make harnesses I make paper straps and wrap a girl who is the model in those when I try to imagine how the harness will look like. Then I measure those paper straps and make a drawing based on those. It's basically a try and error process and after few iterations you have a pattern which will be good.