Score the tree around the branch in two places a couple of inches or so apart.
A branch no thicker than a pencil would be a better choice. I am doing this larger one in an attempt to save a badly damaged pear tree.
If it is a small branch, you can use plastic wrap.
Check frequently and keep moist.
Roots will grow. Once a good root system has developed, you can separate it from the tree and plant. This may take many weeks.
Update 4/17/2016: Yes, this worked! I have used this technique several times. I am currently using it on poplar trees. If you are going to prune or cut it anyway, why not try to get another tree out of it? If not for yourself, then to gift to someone else or to a little extra money.