A written instrument is legally sufficient under paragraph (1) of Section 5 if the wording of the instrument complies substantially with Section 10, the instrument is properly completed, the instrument is signed by the decedent and the agent and the signature of the decedent is notarized. The agent may sign at any time, but the agent’s authority to act is not effective until the agent signs the instrument. The written instrument may be modified or revoked only by a subsequent written instrument that complies with this Section.