What can be done if there are doubts about the Testator's testamentary capacity (e.g. not of sound mind)?
If the Testator proceeds without addressing such doubts on his testamentary capacity, it may lead to complications down the road (e.g. contestation on the validity of the Will).
The certification document does not need to be in a specific format, as long as it is signed by the certifying party. The document can be attached as a supporting document to attest to the Testator's testamentary capacity. Although it does not form part of the Will, it should be kept with the Will as evidence should any doubts arise regarding the Testator's testamentary capacity at the time of signing their Will.

While the certifying party (who certifies that the Testator has testamentary capacity) doesn't need to be one of the witnesses of the Will, it will be good if possible.
Important Note:
For Wills that appears to have been signed by a blind or illiterate testator, or if there is some doubt about the signing of the Will, the Singapore court may require a supporting affidavit (i.e. affidavit of due execution) to be filed during probate application after the testator's death. The law firm assisting with the documents for probate application can help prepare this affidavit. You can read more about the affidavits requirement in the Family Justice Courts Practice Directions.