I was taught to FIRST do the lap belt up nice a snug, low on the hips, making sure there was nothing, other than a layer of clothing, between belt & skin (no keys, pocket knife, wallet etc)- then do the shoulder straps snug. My shoulder straps have a quick release tab, so that I can loosed one/both in flight to reach across the cockpit (retighten after job done). Check again for tightness before landing.