Well, I don't. I think it's an incoherent concept. On a classical level, the universe is deterministic, it follows cause and effect and we are a part of such construct. Not even quantum mechanics, with its probability framework, escapes determinism kick-started since the Big Bang.
In fact, even if one posits the existence of a soul, the notion of free will is still incompatible. The soul's urges to do something are just as mysterious and certainly dependable on what happens environmentally and how one feels about it. Likewise, a soul is also no explanation for a life force or consciousness. One would also have to ask how is a soul conscious? It does not even begin to address the problem.
So, you are as free to decide your next move as you are free to decide what card you will play next. The decision-outcome is constrained by the rules of the game, how things pan out, how one feels about how the game is going, and, of course, the hand that one has been given (which one did not pick).
You did not pick your genome, your gender, your family, what happens to you, or what country you are born. All of these things are beyond your control and you merely behave accordingly because all that you are is atoms and molecules following cause and effect. Free will is an illusion.
Someone who is a manic depressive cannot help this, due to his brain states, and this may lead to his decision to end his life. But this is not a decision from free will. It is an urge that one could not help. Brain states dictate that the will to die is stronger than the will to live in this case. We have will, but it isn't free.
As neuroscientist Sam Harris once put it: "You can decide what you decide but you cannot decide what you will decide."
As the late intellectual Christopher Hitchens once quipped: "Yes, we have free will...because we have no choice."