If you want to exercise a self-employed activity  in Belgium as a non EU national, you must first obtain a professional card (Carte professionnelle/Beroepskaart). The application is examined by Brussels Economy and Employment (The Regional Ministry for Employment) and judged by a number of criteria, like the relevance  of the project in terms of economic utility The application must be in principle made from the Belgian embassy in your home country but can be lodged from Belgium if you hold a valid residence permit.

For the conditions and the procedure click here.