Contact a representative involved in the papers you're signing. For example, if you need to sign a check, contact a bank officer. If you're signing for a real estate transaction, contact one of the attorneys involved. Ask what method of signing is preferred.
Sign the documents as you are instructed. If you don't have a representative to ask, check state laws regarding power of attorney signatures.
Use standard wording if you're unable to confirm how you should sign. A common format is signing the principal's name and writing, "By (your name), as (agent or attorney-in-fact)." Another option is signing your name and writing, "As (agent or attorney-in-fact) for (principal's name)."