Most people don’t know how to wear their large waterproof backpacks. The vast majority of people will throw their bag’s straps onto their shoulders and simply strap the hip belt where it sits. This is not the right way of doing things.
The whole point of the hip belt is to relieve your shoulders of the weight of your bag. If you strap the belt across your stomach it will actually make the bag feel heavy and could potentially hurt you.
To attach this strap properly you will have to find your hip bone. This will usually be where your pants rest on your hips. When you sling your bag over your shoulder you may have to lift it to get the strap situated in the right place. You can do this by resting the bottom of your bag on the top of a table while making sure the hip belt lines up properly.
Once the belt is securely tightened you can lift it off the table and adjust the shoulder straps to secure the top to your body. If it is adjusted properly you shouldn’t feel a lot of weight on your shoulders.