The most popular approach is to line your empty backpack with a large tough plastic garbage bag or two (the strong orange ones seem to be good) or commercially available pack liner or dry bag. Then everything simply gets packed into this as per normal.
Barrington Tops National Park, NSW Notorious for needing to waterproof your pack!
how to waterproof your backpack:This method has some advantages, especially if you know you’re going to be canyoning or using your backpack as a float/pack-raft along rivers. When you go to seal the liner, you can make sure that there’s a good amount of air trapped inside which will aid buoyancy.
Another approach is to use a Pack Cover like the one in this photo taken in Barrington Tops National Park during my Tops to Myall Heritage Trail trip.
However, for most trips, I use a method that sees the individual items waterproofed. (For me, I find that the all in one liner bag tricky to negotiate, whilst still ensuring a tight pack to my bag.)
how to waterproof your backpack?
So in my approach, I have my clothes in a lightweight Drysil bag, my First Aid Kit is inside the waterproof plastic containers and all my food is in Ziploc bags. Therefore, the only thing I need to waterproof is my sleeping bag. (If needed, I can put my matt inside a Ziploc bag also).
So here’s a video that shows you a little trick about how to waterproof your sleeping bag.