There is a right way and a wrong way to put on your loaded backpack, and you'll know by the end of a day of hiking.
The right way carries the load on your hips so you can use the big muscles of your lower body. The wrong way carries the load on your shoulders and puts unnecessary pressure on your spine, which causes a host of issues you want to avoid.
When you're fitting a pack, first make sure the harness is adjusted to fit your torso length. You may need to move the shoulder straps up or down - any reputable pack brand will have this adjustability.
Once you do this you won't need to adjust it again. Every time you put on your pack, simply follow the four steps below - in order - and you'll be carrying your load correctly.
Step 1: hip belt
First, loosen all the straps on your pack. Then find the bony crest of your hip bones. You want most of the weight you're carrying to sit on these bones. Position the hip belt over your hip bones and tighten the straps. You'll want them quite tight.
Step 2: shoulder straps
Next pull the shoulder straps down and back until they sit comfortably flush to your body. They shouldn't be as tight as to lift much weight off your hips, just take the slack out.
Step 3: load adjustment straps
Tighten the load adjustment straps gently to pull the load closer to your body so it's easier to carry. Again, just snug. If they pull your shoulder straps up off your shoulders they are too tight. You want the curve of your shoulder straps to contact you all across the top of your shoulders.
Step 4: sternum strap
Finally, make sure the sternum strap is positioned correctly: an inch or two below your collarbone. Snug it up to pull the shoulder straps slightly into a comfortable position. Not too much! You don't want to distort your shoulder straps or you'll get pressure points. We see lots of hikers with their sternum straps pulled way too tight.
You are now good to go. You should feel most of the weight on your hips and the first few steps might feel a little off balanced. You'll get used to it.
As you hike, you're going to get sore. Experienced backpackers will micro-adjust their straps many times during a day to shift how the weight is carried. If your hips get sore loosen your hip belt a little and tighten up some more load onto the shoulder straps for a short while. Play with these adjustments to see what gives you relief.
When you take your backpack off (and do this every time you stop, to give your body a break) simply go backwards through the steps to loosen everything.