(Incidentally, I've been using Alex Schroeder's Gridmapper for these quick demonstration pics. I only recently found out about it, but I've been out of the loop for a while, so probably everyone already knows about it. I work on the pic in another tab as I'm writing the blogpost, then use a Chrome extension called Nimbus Screenshot to quickly save the pic and insert it into my post.)
An alternative that might be easier for some people would be to mentally overlay a tic-tac-toe board over the map of the area immediately around the starting room, with the center square as the current room, and number the other squares clockwise from 1 to 8, starting at the top. Use a d8 with this method to indicate the destination square.
In either case, the corridor heads towards the indicated destination. If the area hasn't been mapped yet, you can roll to see what is in the area using a d10:
- On a 0, the passage dead ends.
- On any result up to 6, there's a junction. Roll again twice for destination with either the d12 or d8 (whichever method used above,) with doubles indicating that the junction is a turn rather than an intersection.
- On any result over 6, there's a room. Roll on any table you prefer to determine room size, shape and description.
You can change the target number of 6 to a different number to shift the proportion of corridors to rooms, or use the junction table on the pre-loaded non-linear dungeon post instead.
If there is a room or corridor already in that location, the new corridor connects with it. (Notice that we don't need to connect with a room, or even with a keyed location...) Alternatively, for corridors, use the d10 table for crossing pre-existing corridors from the SubMethod I post, which will add a chance for sublevels above or below the main level.