This tutorial assumes that you can already make an individual room.
Included: corridors.map (the final .map file for this tutorial)
Start off with making a simple room:
Now select one of the walls and use the Clipping tool , cut a hole in the wall in the shape of the corridor you want.
How to use the Clipping Tool:
You should end up with something like this:
Now create an identical room but facing the opposite direction (you can select all the brushes in the room by shift+click, then duplicate with 'space', and finally rotate with ):
Using a similar method to what you used to make your initial room; create a long, open-ended room (ie 2 walls, ceiling and floor) that fits the holes in your main rooms:
(Note this is a view from the side)
Now you should be ready to add spawn points and lights for a test compile. Here is a screenshot of the level running.