Since there are so many agencies operating in the this area, there is no one way to PCS into Brussels. There is however a good amount of useful information available on the web to give you a good start. What is offered here are links to some of those sites.
Brussels Garrison Contact Info
A Guide for Helping Kids and Their Families During PCS Moves
SHAPE Newcomer's Guide - Lots of great info on coming to Belgium
80th UASG - The Best of the Benelux (external link)
Brussels - Official City Site (external link)
Military Child Education Coalition (external link)
US Embassy Guide - Living In Belgium (external link)
Visit Belgium - Official Tourism Site (external link)
80th UASG (external link)
Two expatriate sites (external) on Living in Belgium, Classifieds, News, etc.
Isles District PCS Page (External Link)