PP is an abbreviation for the Latin phrase "per procurationem." Per procurationem means that you are signing a letter or other document on behalf of another party. It is widely used in business situations where a secretary/personal assistant is authorized to sign documents on her boss's behalf. It can also be used in personal matters where an individual who has power of attorney over another needs to sign a document on his behalf. A person can only legally sign for another if he is given the proper authorization to do so.
Make sure you have the authorization to sign the document on behalf of the person you are signing for. If you do not regularly sign on behalf of the person, get her permission first.
Type the letter as you normally would with the information of the person you are signing for. If you are not personally writing the letter you can skip this step.
Write pp and then sign your name in the signature spot designated for the person for whom you are signing for.
Deliver the letter as usual.