My opinion is that you should never simply tie a bear bag up in one tree. Bears and a number of other
critters can climb trees and still get at your food.
The technique I will describe for tying up a bear bag may be more involved, but is probably more secure. And which would you rather have, an easy time of tying your bear bag, or breakfast?
You will need your bear bag, your rope and a good odd-shaped rock or stick. One person can do this; but it goes faster if two work together.
Find two trees, about 30 feet apart, with no other trees between them. Make sure the trees have nice branches about thirty feet up.
Tie the rock or stick to one end of your rope. Throw that over one of the branches in one of the trees, as high as you can. Now do the same with the other end of the rope.
You should now have your rope strung between the two trees, with the middle of the rope on the ground between the two trees. Tie your bear bag to the middle of the rope.
Now, if you have two people working on this, each person hauls on each end of the rope, thus suspending the bag between the two trees. Secure each end of the rope to its respective tree.
If you are tying up the bag alone, you will have to do one end at a time, but the principle is the same. The bag should be suspended between the two trees, as high off the ground as possible.
That's it. I was taught this method by a more experienced backpacker and I trust it. It occurs to me that if a bear chews the rope in one tree, the bag will swing down to the other tree. If you have picked good trees, you might be able to rig the bag such that it is still high above the ground when it falls. Then the bear has to chew the other rope. By this time, you should be awake and scaring the bear away from a minimum safe distance.