Refer to your state statutes or notary division. Many states allow "Signature by Mark" (the making of an X or some other mark) but are very specific on how this type of notarization has to be handled. There may be special notarial certificates that need to be used in these type situations. Witnesses will need to be present and it is prohibited or recommended that the notary not be one of the witnesses. Notate this in your notary journal.