I take fall arrest classes for my work.
Best advice i can tell you is to wear it correctly with the leg straps snug as in the event of a fall the forces applied will yank them into your groin, if not used you risk falling out of your harness
chest strap should be across your chest centers over your nipples and snug but not super tight
lanyard should be hooked to your harness and be between your shoulder blades.
other end of your lanyard should be anchored a high as possible to limit the distance you can fall before your system begins to work.
if you dont have a way to get down in the event of a fall, look into getting some trauma straps for your legs to allow your legs not to pool with blood, after 15 minutes of hanging your odds of surviving are greatly reduced.
let people know where your at and keep a cell phone handy in the event of a fall
if you do fall and hang more than several minutes if you are able to get down dont take off your harness right away, can allow the unoxygenated blood to overwhelm your heart.
would recommend you also seeking medical help as you can suffer internal damage from the forces of falling.
harnesses and lanyards are generally 1 use only items. I would recommend cutting the straps to avoid being able to use it again and trash it.