Somebody more knowledgable than me will probably be along with the ultimate guide just now, but here's how I've been doing it, successfuly, for years; thinkk of filling the volume of the pack, and aim to get a fair but not heavy weight into it, hoist it aboard and get the shoulder straps adjusted so it fits your back and the hip-belt sits comfortably over the hips, where it will be able to perform it's function of transferring some of the load down through the lower body. Fasten the hip-belt as tight as is comfortable to perform it's function, and when this is satisfactory adjust the suspension syraps to get the load as close to the body as possible.
It's important to load the sack properly for any of this to work, and there's a good sticky on here to clarify this........................hope this is helpfull. atb cake.