Preparing and executing your codicil follows the same rules as you would have followed when preparing your last will and testament.
In order to make your codicil legally valid, you need to ensure that it is properly executed. This means you must sign your codicil in the presence of two witnesses (three witnesses are needed in New Hampshire and Vermont). You must be at least 18 years old to sign the will (16 years old in Louisiana and 14 years old in Georgia). Also, you should avoid the appearance that you are being pressured to execute your codicil according to someone else's wishes or that your true intentions are otherwise being restrained.
You should choose people who are disinterested parties to witness you sign your codicil. Note, if you are unable to physically sign the codicil, then you may direct another person to sign for you. This means that you should not use as witnesses any person named in the codicil or the will it is altering. You also should not use any relative, since a court will probably find that they have an interest in your will whether they are named in it or not. Finally, the witnesses should be over the age of 18 (age 16 in Louisiana and age 14 in Georgia).
After the codicil, you will find a document called "Self-Proving Affidavit." This document is useful for speeding up probate of the will by helping to establish that the will was properly executed and witnessed. Note that courts in the following states are not likely to accept such affidavits: Ohio, Maryland, District of Columbia, and Vermont.