Single rope thrown over a high limb. Tie one food bag to one end, and pull it by the other end of the rope. The food bag goes up, and you still have the pull end of the rope in your hand. What are you going to do with that? You have to tie it off to a nearby tree trunk. A black bear comes into camp during the night, sees the first rope it can find, scratches that until it breaks, down comes the food bag, and the bear grabs the food and feasts.
Double rope method. One rope is thrown over the high limb the same way, and one food bag is attached to the thrown end. The second rope is -looped- around that food bag and left dangling. That food bag is hoisted up to the limb by pulling on the other rope end. Then you reach high on that pulled rope end and form a loop for attaching the second food bag, with the remaining pulled rope end stuffed loosely into or around the neck of that bag, with only two feet or so left out. In that two feet, you have a bowline loop, and it hangs down ever so slightly below the second food bag. Now go back to the second rope that was looped around the first food bag. There are two ends of it hanging down, so you pull evenly on both ends together. That pulls the first food bag down halfway, and it pulls the second food bag up halfway. When both food bags are halfway and even, you release one end of the "pull-down" second rope and then pull on the other end. That removes the entire "pull-down" rope and the two food bags are left dangling high overhead with no "tie-off" rope to any tree. Mister Bear can't find any rope to scratch at.
In the morning, you grab a loose tree branch, raise it high, and hook the rope loop that is by the second food bag. You pull it down, then pull the second food bag down to release it from its rope. With that weight off, you can lower the first food bag down. Leave the rope on the limb if you need to spend another night there.
If this is not clear, I probably have a diagram that I furnished to the National Park Service about 25 years ago.
Advanced techiques: Use dark ropes and food bags. It makes it harder for the bears to find at night. The bears can smell the food, but they have a harder time trying to figure out which trees to climb and which limbs to attack.
Take an ordinary tan paper grocery bag and string it up with some bright white cord, and tie it to some nearby tree as a decoy for the bear. The bear attacks it first and makes some noise, thereby giving you a minute to wake up and defend the real food bags.