Are you getting travel insurance? If so, check the small print. Many won’t cover you unless you have a return ticket paid for before you leave. There are ‘nomad’ type policies so you could look for one of those.
Book an appointment, turn up an hour early and fill in the forms. Go to the reception desk to check that you did it correctly. When this is confirmed, go to the immigration officers and wait your turn. The actual appointments don’t seem to be used. Don’t forget the photographs and the 1900 baht in cash.