Can a police officer sign as a notary?
Not unless he is a registered notary public.
No it has to be a registered notary. Most real estate offices have one or more that will notarize for a fee. Added: There are no laws prohibiting law enforcement officers from becoming, or functioning as, a state commissioned Notary. However they WOULD actually have to be state commissioned. Police officers are not automatically granted Notary powers simply by virtue of their status as sworn law enforcement officers.
Notaries don't notarize documents, they notarize signatures. A notary is simply certifying that the signature on the document is actually the signature of the person it appears to be. The notary makes no warranties or certifications about the contents of the document or what it does. A notary rarely even reads the document. Instead, he/she will look at the signor's identification, watch them sign, and then sign and stamp as notary.
It depends on what you're asking. I suspect you mean, "How do you get a document notarized?" If so, you take the document to a Notary Public, sign the document in front of the Notary, and show ID. The Notary will stamp, date, and sign the document. You can usually find a Notary at a local bank. A Notary will normally charge you a small fee.