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!!!
If you take off "one layer, one jacket, one soft shell" during the course of the via, remember to re-tighten your harness.
Before attacking the via, have someone else make sure that you are wearing your harness correctly and that it is closed. To be extra cautious you can let yourself hang in your harness at the start of the via to make sure everything stays in the right place. Clip your lanyard on the cable and sit back in your harness.
Full body harness
Full body harnesses are not recommended at all anymore (can lead to a whiplash injury in the event of a fall). These harnesses were especially designed for children so that they would not slip out of them.
But with modern day harnesses and tightening at the waist, it is preferable not to use full body harnesses!
Ex: Full body Harness: Not Recommanded