Go to the retirement desk at the Krabi immigration office and ask them. More than likely they have a preprinted list (in English) of what you will need for your extension. Then return on the day you plan to actually do your extension, with all of your affairs in order.
I wish you good luck. Getting your non-US spouse a green card to live with you in the US was challenging before the Dear Leader became POTUS on Jan 20th. I'd wager it's damn near impossible now.