Hi Murph,
Firstly, the boom should be around shoulder height when sailing normally.
Getting the harness line position right requires a bit of trial and error, but when you've got it sussed you should have an even pull on both hands, and should be able to let go the boom with both hands and sail along. If you find you're pulling too much on the front hand, move the harness lines forward, if back hand has too much pull then move the lines backward.
The two lines should be around a handswidth apart on the boom, this is a nice compromise.
Hooking in can be a bit tricky at first, basically you do a big pelvic thrust so the hook grabs the lines. Lean back, and take some weight off your arms. Sometimes if the lines are too low then you do a sort of sideways grab so the hook grabs the side of the harness loop, then sit back and the hook slides down to the bottom of the loop.
Good luck, give it a go, you won't look back

Edit: Forgot to add, learning the harness will involve many catapults. This is where the sail sheets in and pulls you violently over the front of the board, still hooked in. It can be painful, and can damage the nose of your board. If you can get your hands on a boom bra then this helps to limit the damage to the nose in a catapult.
To avoid catapults, keep your back foot well back on the board, without sinking the tail. Also be prepared to unhook from the lines while you're underwater and underneath the sail. The first time this happens there's a tendency to panic, but just think it through slowly and you'll be fine.