Plan A is let nature do it. This might take a season or two. You can let it season “in place”, or cut it and store it somewhere out of the way. Sometimes the dead tree is already down, and just seasons in place. In that case, one can then cut it up and split it if need be, and then use the results to move to a log pile.
Plan B is one link on the subject: http://www.wikihow.com/Season-Firewood