Review the original will. Your codicil should identify the specific portions of the will it is altering by quoting the exact language from the will and citing directly to it. Hopefully the original will is divided into sections and the paragraphs are numbered to make citation easier. Failing that, merely identifying the changed sections using page numbers will be enough.
Draft the codicil. For a codicil to be recognized under probate law, it must be drafted under the same guidelines as a will. It must be in writing, reference the original will by the date it was executed, and explicitly state that you intend to amend your will. Clearly define the changes that you are making to the will by referencing the sections of the original will that are going to change and by using clear and definite language. Avoid legal jargon.
Sign the codicil in the presence of witnesses. The witnesses must see the drafter sign the codicil, or the drafter can tell the witnesses that the signature on the codicil is his. You can satisfy the witness requirement by signing in the presence of two people who are over 18 or in the presence of a notary public. The witnesses must sign the codicil. If the witness was a notary, he must affix his seal to the codicil.