Verify that you are old enough to make a will in the jurisdiction and that you have met all other qualifications. Generally, testators must be residents of the state and of sound mind.
Review your will. Assure yourself that it lists all of your assets and designates beneficiaries for each. Check to see that you named a guardian for your minor children, if any, and appointed an executor, charged with gathering and distributing your property. Note whether you named a residual beneficiary -- the person who will inherit anything not specifically bequeathed in the will. Check for will language stating that this will supersedes all prior wills.
Research the witness requirements for a last will and testament in your state. (Most states require the will be signed before two impartial witnesses.) You can usually find this information on your state's website.
Affirm your testamentary intent and sign and date your will before the correct number of witnesses. The witnesses must be of-age, of sound mind and impartial -- not heirs under the will they are witnessing. Will signatures generally do not need to be notarized.
Check with a lawyer if you have questions or doubts about the validity of your will or special estate issues.