@Randula **************
I forgot to explain, the process is very simple. Find the section that prepares the residency letter, give them your documents, they will check them and you pay the fee. Hua Hin and Sri Racha issue the letter within a few minutes, but some offices post them out the next day. I always go when I think it will be quiet, maybe 2pm on a Thursday (something like that). You need one of these letters for other things too, like opening a bank account or buying/registering a car/motorcycle. Good luck.