Belgium is full of ’em, Kermesse is the word you want to look out for.
I’ve done plenty of cross races but never a Kermesse so don’t know the exact in’s and out’s but I think you’ll need a Belgian registration card, usually available at the race for a few euro’s and an International Racing Licence from British Cycling.
Normally (with cross races) you just rock up at the (usually) local cafe the HQ will be in, pay your money, sign on and race. normally money down then deposit back when you had your number back and if you’re lucky an envelope of prize money. Categories are usually Elite Zonder Contract (Non Professional Elites) then lower cats A,B,C etc.
Don’t expect an easy ride though, Belgians will kill you for a tenner and tend to ride like it’s the last thing they’ll ever do.
I’ll try and find out the script though as a mate of mine is a Pro in Belgium so will know the score.