Make sure you make walls which are light portal-able and walls which are not dark, like in the game. Most of the custom maps I've played sofar were covered in non-portalable light shaded walls.

We're trying to come up with a method to use the dark areas correctly at the moment. Right now, the problem is that all surfaces can have portals on them. Just pretend you cant shoot a portal on the dark areas, for now, when you see a map that does that. There have been a few fixes, but still, i dont enjoy how they work as they increase the size of the maps quite dramatically (when i do it, at least).