The codicil must be signed with the same signing formalities as the original will, and should be kept with the original will. Your "will" is now your original will, plus the codicil.
Usually, codicils are used only for minor changes, for example, to include the name of a new executor to replace a formerly named executor who has died. If you want to make substantial and/or numerous changes to your last will, it may be preferable and less confusing to simply prepare a new will.
This article contains general legal information and does not contain legal advice. Rocket Lawyer is not a law firm or a substitute for an attorney or law firm. The law is complex and changes often. For legal advice, please ask a lawyer.