Custom levels are built using a point system. Points are charged for each item used to populate a level (examples: public trash can, 2 points; blue elebit, 3 points; toy tank, 28 points). No level can exceed 1,000 points.
Once completed, the level can be 'overnighted' to registered friends, along with choice screen shots. It's as of yet unclear if friends can further edit your creations, which appear to be limited by pre-made environments (e.g., bedroom, yard, street, etc.). If level building is this restricted, then one has to question this feature's longevity. Here's to hoping Konami is dedicated to implementing some bigger possibilities for WiiConnect24.