DoomEd - How to do it:
#3 - Create a door:

   +---------+      +----------+
   |         |      |          |
   |         | +--+ |          |
   |         | |  | |          |
   |         | |  | |          |
   |         | +--+ |          |
   |         | door |          |
   +---------+      +----------+
    sector1           sector2

When you right click on a line with the Vertex tool,
it breaks the line and lets you drag around a new vertex.
You must add four vertices:
 
   +---------+      +----------+
   |         |      |          |
   |         O +--+ O          |
   |         | |  | |          |
   |         | |  | |          |
   |         O +--+ O          |
   |         | door |          |
   +---------+      +----------+
    sector1           sector2
  New vertices indicated with "O"

Now, drag and join the vertices thus:

   +---------+      +----------+
   |         |      |          |
   |         +------+          |
   |         |      |          |
   |         |      |          |
   |         +------+          |
   |         | door |          |
   +---------+      +----------+
    sector1           sector2

Then, with the Sector tool, right click on your door.
Choose the "Make sector a door" option.

Also, you could combine some of these steps. When you create a vertex,
drag it into position while still holding the right mouse button.

(Geoff Allan)
