How do I become my moms healthcare power of attorney. She is past the stage of signing?

Too late, but. If you are closest living relative, healthcare questions might be deferred to you; however, legally speaking, if there are other siblings /close relatives that have differing opinions, it's problematic. Every person should declare and sign (1) a health care proxy, who can make medical decisions in case the person is not able to. (2) a will. Also consider (3) durable poa / financial poa.