Forms of will in Thailand are prescribed by the section 1655 to 1672 in Civil Code of Thailand. The most common type of Last Will in general in Thailand is an ordinary will according to section 1656:
“A will may be made in the following form, that is to say, it must be made in writing, dated at the time of making of will and signed by the testator before at least two witnesses present at the same time who shall then and there sign their names certifying the signature of the testator. No erasure, addition or other alternation in such will is valid unless made in the same form as prescribed by this section.”
The above mentioned form does not have to be notarized or authenticated but must be properly witnessed. You can also choose to register a will (public or secret) with the government (the District Office) which requires Thai language skills and a yellow house book. We advise you have a lawyer to deal with Thai authorities.