@Nathalie *******
You can just turn up. No appointment needed. TM30 is not needed for a tourist visa/exempt entry extension.
Passport, copy of Photo page, copy of entry stamp (both signed by you).
Passport photo. 1,900 baht.
Fill out TM7 (available on site).
Depends on how busy but bank on around 1 hour.
Morning queue closes at 11:30. Afternoon queue closes at 3pm.