Have your power of attorney document with you when you sign anything on the principal’s behalf. The entity or person with whom you’re contracting will probably want proof that the principal has authorized you to act for her. Ideally, the principal has already provided copies to all institutions with whom she expects you to deal, but don't count on this.
Sign the principal’s name first, not your own. This eliminates any confusion that you’re acting in your own interests or assuming any personal liability for what you’re signing. The principal is actually the one engaging in the transaction.
Sign your own name after the principal’s name, after including the word “by.” This indicates that the principal is engaging in the transaction through you. For example, you would write, “Sally Smith, by Samuel Smith.”
End the signature by indicating that you’re acting under power of attorney. You can do this in one of several ways. After your name, you can write in the words “agent,” “attorney in fact,” “power of attorney” or simply, “POA.” Your final signature should read similar to "Sally Smith, by Samuel Smith, power of attorney."