i can't speak on the result yet (since i haven't received my jacket) but i think it's fairly easy with a measuring tape
it definitely helps with buying other clothes online provided that the retailer has garment measurements. just follow the tumblr's guide. but of course if you're really particularly picky about it, go to a tailor. i think charly mostly based his recommendation of measurements off my levi's trucker i measured instead of my body though.