What is the best way to make a simple Legal Will here in Thailand.

No property, just personal belongings.

Perhaps this will help:

http://www.thaivisa.com/forum/topic/618983-the-importance-of-preparing-a-will-in-thailand/

If the personal belongings are all located in Thailand, then a simple will would be fine. I have a sample if you need one - just send me a PM.

If any of the belongings are located in the US or elsewhere, then a will executed in Thailand may not suffice, depending on individual state laws, and you would need a will will that meets the requirements of that particular state.

You need two wills, one for each country where you have assets.

In Thailand you can do an Amphur will which will cost you 50 baht and appropriate if you have limited assets and your will is unlikely to be contested.

To do an Amphur will; you can have the Amphur write it for you based on what you tell them and they will supply witnesses to your signature. Alternatively you can do a secret will whereby you write the will and give them the document in a sealed envelope. Either way they will file the document and give you a receipt which must be produced in order to retrieve the will.

The cost to write a will in Chiang Mai, using a decent solicitor, is about THB 5,000.

EDIT to add: the will should be written in your language and translated into Thai., not the other way round.