What is the process for extending a METV visa in Bangkok?

I missed the part that you wanted a step by step guide to what to do once you're there.

Get your documentation together, go in and show the people at the queue counter your passport, they will give you a queue for section J-2 (the section that does extensions for people on tourist visas), go sit down in that section until your queue number is called. When they call your number go give the officer your paperwork and passport and 1900baht. Wait. they'll have you go sit down and call you when your extension is done.

You will NOT be the only foreigner there doing this process, and you need to NOT over complicate or over think it.

Sorry your topic was closed without you getting your answers,

BUT

you could have messaged one of the admin staff rather than posting on Bangkok Expats that your thread was closed without you getting your answers :/

You can go to the copy shops in the basement of the government complex where the immigration office is and they will make your copies, fill out your form, take your photo if you need.

copy id page passport, copy visa page, copy Admitted to Stay stamp, copy TM 6 departure card, TM 7 Application form, 1.900 Thb fee.