The first one to get right is the back length; loosen everything off (so you can wear it but nothing is tight) and put the pack on and see how it sits. You want the waist straps to sit just above your hips. If they don't adjust the back length (up or down). Once this is set don't alter it.
The other straps will likely alter depending on your clothing and what your carrying.
Shoulder strap length/Chest strap
I'd put it on attach the chest strap and pull this into position, then tighten the shoulder straps so that they're tight(ish). Then tighten the chest strap, you want the shoulder straps slightly off your shoulders so that some of the weight is distributed onto your chest slightly, this prevents heavy weight's pulling your shoulders too far back. The weight should be evenly distributed into a H shape across your chest and shoulders with no one part (one shoulder or just your chest or just your shoulders) holding more weight than the other areas. This evenly distributes the weight.
You shouldn't feel pulled into any particular position, if you are they're probably too tight.
The last strap to adjust is the shoulder length (the strap behind your shoulder). This is designed to slightly pull your shoulders back and evenly distribute the weight across your back (i.e. so that the weight doesn't simply sit on your lower back). Pull these tight so that you feel the weight move up slightly (to the middle of your back) and doesn't sit too low. You should feel your shoulders being slightly pulled straight and backwards (imagine your shoulder blades being pulled flat with your back, this should naturally adjust your posture so your shoulders are flatand your back straight).
The back length dictates this really. You can alter the tightness but that's it really. It should be snug but not tight. Waist straps can distribute some of the weight from your lower back to your hip area. they also prevent the bottom of the bag from moving around
You will likely adjust the shoulder (not length) and chest strap often, but if you get the others right these shouldn't need to much adjustment at all. Adding and removing clothes alters your bulk so you will normally need to adjust your pack too. Adding and removing weight will also naturally require some adjustment. Try not to fiddle too much though, get it into a good position initially and try and maintain this position.