How to get power of attorney for insane person

Is this person a family member, a friend, a stranger? What's the situation?

An "insane person" (person who does not have the mental capacity to make basic financial decisions) cannot create a power of attorney. And you cannot do one in that person's behalf, "sane" or not. What you would have to do is petition the court to be appointed that person's guardian, conservator, or whatever they call it wherever you are.