Go to Promenada any time after you arrive. Immigration is in the parking garage. You will need a passport photo and a copy of your passport info and arrival stamp pages. You can obtain those at a little shop right there next to immigration. Fill out the extension form, front and back. It will be in a basket at the front of the immigration lines, outside, you'll see it. Wait in line with these and 1900 baht. After they review your papers and take the money, you go inside and sit until you are called for a photo taken by them, then you will be called again to retrieve your passport with the extension stamp.