Well, that depends on the mutation, doesn't it? There's any number of ways one can happen. Damage might be caused by an external source, say, radiation, which I highly recommend avoiding as the process can be quite painful or perhaps a virus, even a prion. Then again, the damage might be something of a genetic typo. All of those macromolecules getting copied and rewritten and spell-checked and, occasionally, your cells get it wrong, and something new is created, something that may or may not work. Animals are remarkably bad at adjusting to true mutations, you see. Plants really do a much better job than any of us. Still, you occasionally get a change that works and that sticks. Take lactose tolerance, for instance. A rather large portion of Earth's population has it, but almost none of them realize that simply being able to drink milk without experiencing some gastrointestinal discomfort effectively makes them mutants.