Choose a harness that is adjustable at the thighs and at the waist. For luxury and comfort pick one that is padded!
Start off by tightening the harness at your waist (be sure it's closed correctly) and then you can adjust the legs.
Close all the buckles vigorously. There should not be too much empty space between your body and the harness. The tighter it is, the better you are protected. But of course you still need to be able to breathe!!!