You just need to provide proof of your income (mine was NZ super and military pension) and they provide a letter confirming the annual amount in Thai baht. When I did my extension I attached a copy of both pension letters and a couple of my NZ bank statements highlighting the funds going in as proof.
You can email the Embassy at
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and they'll tell you what they require, cost and bank details to make payment (was 900 baht last time I did it). Takes about a week tops, they should email you to confirm the letter has been sent (they will attach a copy with the email) and tracking number. Hope this helps.