Make sure the document is clearly referring to you and could not be mistaken for another person of the same name. This means adding unique identifiers such as date of birth, social security numbers, home address, driver’s licence number, national or provincial ID etc.
Make some declarations about the document as being your last will & testament, it is being made by you willingly and you are of sound mental health and of capacity to make this contract.
Identify this will as the current valid will & testament and that it nullifies any previous will created by you or by anyone for you, prior to this one.
To prevent challenges to the will, you need to state that you are of sound mind and not suffering from a mental ailment affecting your choices. You should state you understand the effects of the creation of the will. Additionally, it is highly beneficial to supply evidence to back up your statement such as a video testimony or a medical evaluation and refer to it in the will.
Further challenges to will typically include claims of will created under duress. To exclude this, you also need to declare that you are making the will free of undue influence or duress.