I make the player word out their wish and then interpret it as literally as possible. But since the players are my friends, if there's anything egregious in their wording that would result in them not at all getting what they wanted, I generally tell them out loud "this is how I would interpret that wish if you submitted it as the final version." I don't necessarily want to give them too hard a time over it, its just enough power that I want them to at least try and get a little clever with it.