This isn't based upon however they ask you to do it in the editor, this is just the rams way.

Mash your rooms, make one between that will become a door.

Select the linedef that's facing inwards, and hit "F" to reverse it.

Texture shit.

Select both door sides and right click. Check off "Lower Unpegged" and then hit ok.

Select the door 'room' and make the ceiling height the level that the bottom of the door will be when closed.

Texture more shit.

Select both linedefs on the outsides of the door, and right click. Click "Select Action".

Expand door and pick one. Hit ok then ok again.

Suck some cock.