I did one three days ago in Hua Hin. I paid 500 baht and got it in 15 minutes. You go to the back row of rooms behind the main building. They are located in room #4. You will need your passport and two recent photos. The room next door #3 can take your photo (8 for 200 baht) and make copies of your passport main page, visa and latest arrival stamp. You then fill out an application form and wait for them to finish.
It is now low season so not many customers when I went. Totally painless.