Go to the local Amphoe office and ask for the requirements. Mine below in Thai. You don’t need an agent just the right documents and witnesses. My local gov office gave me the forms to complete and one was for the head of our moo to complete which meant we did not need him to go to the appointment with us.
I had to show original thai wedding cert, I'm Australian working in Singapore. I didn't have to prove much else other than the head of our Moo signing a paper to say I lived here