I would agree that this is pretty good guidance. Be sure that you do not sign the document until you have 2 to 3 witnesses. These should be people who will not benefit at all from your estate. Sign in their presence, then they sign as witnesses and they should include their addresses on the documents.
Was your original Will filed with the Register of Wills in the county where you live? If so contact that office about adding this Codicil to the Will. Contact the person you name to be your personal representative (executor) and advise them of this document. In fact it doesn't hurt to have two or more originals, one with the original will, one with your personal rep