To fit a loaded rucksack put your arms though the shoulder straps and tighten the straps using the buckles at the bottom of the straps where they meet the rucksack body. Then tighten the tension straps on the shoulder part of the shoulder straps (if your rucksack has them). With the rucksack high on the shoulders fasten the waist belt at a comfortable fit and then release first the shoulder tensioners and second the shoulder straps themselves until the rucksack settles comfortably on the hips.
How you distribute the weight between hips and shoulder is a matter of personal preference and some walkers don’t like their hips pelvis constrained at all. Also, needs vary throughout the walking day and if your shoulders ache towards the end of the walk you may wish to transfer more weight on to your hips.