The most important thing to know is that the MIDDLE strap of your Zena should fit at your natural waist. For boat diving, put your BC/tank on and tighten it down just enough to get off the boat. During your decent, go horizontal and loosen up ALL of the straps, pull down your Zena from the sides and tighten the lowest straps first. Tighten the middle strap, making sure it is at your natural waist or at the very bottom of your rib cage, depending on how you're built. Once your body straps are tight, lightly snug down the shoulder straps. If you've done it correctly, your Zena will never move on our body, no matter what position you are in. Your straps should be about 1/2 used, giving you space for thicker or thinner suits.
For shore diving, I tighten my shoulder straps all the way down first, making my tank as high on my back as possible to walk into the water. Once in the water, I follow the steps above. Then before I exit, I tighten the shoulders again to walk out.
If you want to check the fit at home, put on your wetsuit, put on your Zena with the straps loose. Position the middle strap at your waist and tighten the strap. Tighten the bottom strap, snug up the shoulder straps, and check for fit. If you've used most of your straps and are losing weight, you'll want to look at a smaller size.