1. Load some weight into the backpack so you can get a good feel of how it will fit while it's packed. Then loosen all of the straps and put it on with all of the straps loose.
2. Tighten the hip belt. To do this, you'll want the padded hipbelts covering the iliac crest, or the very top of your hip bones. Tighten the straps until they are snug but not too tight.
3. Tighten the shoulder straps by pulling the straps straight down. Again, don't pull it too tight, you want to make sure your shoulders aren't carrying most of the weight.
4. Tighten the "load lifter" straps. The load lifters help pull the weight of your gear closer to your body. Pull the load lifters until the straps are at about a 45° angle to the backpack's body.
5. Next, fasten and tighten the sternum strap. The sternum strap pulls the shoulder straps together to relieve some of the stress on your shoulders. Keep your sternum strap about an inch below your collarbones.
6. Now that everything is tight, you can loosen the shoulder straps a bit. you should be able to feel the weight of the backpack ease onto your hips. Ideally, about 65% of the weight will ride on your hips.
You should be able to carry your load more comfortably and efficiently when you follow the steps laid out above to adjust a backpack for a proper fit. For more information on choosing the right backpack check out this backpack guide or follow these tips for finding your torso length for the right sized backpack.