I actually stumbled onto portals accidentally in my second LD. I didn't even realize I had done it at first. I had decided I wanted to be somewhere else, turned around, and noticed a door in the middle of the parking lot I was in at the time. The door took me to a small room with another door in it, which took me somewhere else.

Since then, all my portals start off as a brown wooden door in an open area, which always leads to this small yellow room. From this room I can restructure the dream world outside. Then I step through and I'm there. This is the only way I've ever been able to actively change my dreams. I've never been able to alter a dream too severely without waking myself up. Only inside the yellow room can I make major changes to both the "landscape," the "people," and the "rules."