An editorial starting sequence for adjusting a backpack, with the exact model's instructions taking priority.
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Work from the pack’s basic size to its fine adjustments. Start with the manufacturer’s torso range and fitting instructions; straps cannot always compensate for the wrong harness size.
For a backpack with adjustable suspension, REI puts torso adjustment first. Its main sequence then addresses the hipbelt and shoulder straps before load lifters and the sternum strap. Loosen the relevant adjustments before starting so you are not trying to correct a setting inherited from another wearer.
Use a realistic packing load for your own trial, and make each change separately. Once you have a promising setup, put the bag down and try it on again. That reveals whether you can reproduce the fit without a helper holding the pack in position. It is also a chance to learn which buckle or strap controls each change.
Only adjust components the bag actually has. A lightweight daypack does not need an imaginary hiking-pack fitting routine, and a thin waist strap should not be expected to behave like a substantial padded belt. If the best available adjustment still leaves the harness awkward, try a different size or shape before settling for constant readjustment.