OK, if it's a standard TGV, then you're away - just turn up at the station and go. You will be expected to carry on and store your bags yourselves on all these services, incidentally, and there will be no form of check-in (iDTGV and Ouigo do have a 30-minute check-in process, hence asking!)

The Rad Blu in Brussels is by far closest to Brussels Central - a 5 minute walk away!