It is advisable to collect the documents you need to marry in Belgium beforehand. All documents have to be legalised and translated. You will find the necessary information on how to go about this on the website of the Department of foreign Affairs.
Step 1: Obtaining a visa
Partners from a country that doesn't require a visa can travel to Belgium with a valid national pasport. Partners from other countries require a visa. (visa type C)
You find the list of necessary papes and the way to obtain the visa on the website of the department of Internal Affairs & Immigration Office
At the Belgian embassy or the Belgian consulate situated in the country of origin you ask for a visa type C.
Step 2: arriving in Belgium:
In the first three days after your arrival in Belgium, the non-Belgian partner must register an arrival declaration at the city hall of the place where he or she resides.The non-Belgian partner will receive a 'Supplement 3" with which it is legal to stay in the country for 90 days.
Stap 3: The Marriage
the civil marriage is best performed during these 90 days. If this doesn't happen, you can ask the Immigration office to extend the document, but usually administration doesn't do that. The marriage must be performed at the earliest 14 days after the date of the issuing of the marriage declaration and at the latest 6 months after that date. So you can make sure you have the marriage declaration in order before the partner enters the country.
Of course you need some documents to marry. You need: