1) The couple have to handle all legal requirements for a marriage registration and stay in Bangkok for a few days to get all necessary documents translated and approved by both their own embassy and the Foreign Affairs Ministry of Thailand. There are many travel agencies who provide this kind of service.
2) With the assistance of the travel agency, the couple can register their marriage at one of the Bangkok District Office after all.
In case the couple want to register their marriage on Koh Samui, whether to be present at the wedding ceremony or at the Samui District Office, the following steps are required.
A) The approved documents in clause 1 have to be submitted to the District Office on Koh Samui at least 2 days prior to the wedding (not counting Saturday and Sunday or Public Holiday). The District Office will have to check all documents before proceeding with registration.
B) The hotel will charge the optional for this service in case the couple would like the registrar to be present at the wedding ceremony (see the service fee).
C) If the couple do not mind having the marriage registration and wedding ceremony on different dates, they can go to the District Office any day before and after their wedding. The hotel can hlep with the transfer arrangement.