Noobie here. I couldn’t find any guide to it, so I’m listing out what I went through. Hope it’s helpful to other newbies like myself. Here’s what I do. My process takes a long time, but I think it’s worth it. I initially went and just bought a $100 pack because it was on sale. I regret my purchase, because the pack does not fit me that well. spending another $100 would have been money really well spent over the life of use of the pack for more comfort/better fit.

-Determine the necessary pack size. Depending on where I’m going, weather,days out i have pack list. Then I pack it in to one of my existing bags to get an idea of how much volume and weight I typically use for the trips I like.

-go to REI. This is better done during camping season when they have a better selection of packs. Ask an associate to measure you with their tool. I try all the packs in the volume I’m looking for. different models from the same brand have different suspension systems, so I try different models of even the same maker.

-Take the pack over to the baffle bags they have. load a light baffle bag, then weight bags until i’m around my actual carry weight determined in step 1 above. usually i’ll be able to narrow it down to two packs. walk around the store and choose the one that fits best.

-when I had less $, I would then go online and buy the pack since it’s cheaper than REI. I feel it’s important to buy it from the store if you can since you just used their facilities. Or you can just keep in mind the two packs you liked the most, and wait for a sale on either on from REI.

-I take it on a test pack and hike with all the gear. I like to put some real trail miles on. Better to discover the problems now instead of later in backcountry