Traditionally a bear bag was simply something to hold your food out of reach of bears. This usually meant finding a way to suspend it from a high branch, and in this instance it's usually sufficient for the bag to be waterproof. (With this in mind, some people like to hang the bags "upside down" to prevent rain getting in through the top. Be sure to tie them securely closed if you do this!)
There are different methods for hanging, but a common one is the PCT method. To briefly summarize the method:
- Find a branch about 6 meters (20 feet) above the ground. When finally hung the bag should be about 2 meters (~6 feet) away from the trunk, 2 meters below the branch, and about 4 meters (12 feet) above the ground.
- Use a small throw bag (e.g. pouch with rocks) to toss your line over the branch.
- Use a carabiner to attach the line to your food bag, then clip the other side of the line into the carabiner. Hoist the bag until it touches the branch.
- Reaching as high as you can, tie a clove hitch around a small stick.
- Allow the bag to come back down. The stick should be blocked at the carabiner (about halfway between the branch and "as high as you can reach").
There is an alternative to this, and that is the Ursack. A kevlar sack designed to resist tooth and claw, you need only leave it on the ground and loosely tethered to a tree (to prevent the bear wandering off with it).
Finally, you should also consider whether rodents will be a risk. They are quite acrobatic and can chew through many materials. Odor-resistant bags or rodent-proof mesh bags may also be useful.