Don't answer legal questions. Refer the client to an attorney.
Don't backdate. The notarization date documented on the document must be the date of the notarization.
Don't decide what type of notarization to do for the signer. You can tell them what others frequently do, but don't advise them.
Don't notarize the last page of a document only.
Don't allow a page to be added to a notarized document after the fact. You would have to notarize the whole thing all over again.
Don't affix your seal until the signer has signed the document and your journal.
The signer must appear before the notary, or don't notarize their signature.
The signer must be able to communicate directly with the notary.
The signer must have acceptable ID or be identified by credible identifying witnesses if your state allows that. Or the signer can be personally known to the notary (only allowed in certain states) if the notary knows them well.