Many suggest trash/compactor bags inside of pack for "water proofing."

My pack has a access at the bottom for the sleeping bag and other items at the bottom of the pack.
How do you access the bottom items in such a pack? Cut the bottom? Take everything out? Put non-crucial items at the bottom but outside the compactor bag? (Atmos AG 50L) Any other hints using this bag? I have about 75 "practice miles" on the bag.

Thanks