Making a door is a simple process, but may also be one of the first obstacles a person new to Doom mapping runs into.
Suppose that we have a map similar to the one shown on the diagram to the right, with two rooms connected by a hallway. Sector A is a first room, Sector C is the second room, and Sector B will be our door.
Start off by lowering the ceiling height of Sector B to be equal to that of the floor. For example, if your floor height is zero, make the ceiling height zero as well. Make sure to apply upper textures where needed.
Next, select the linedefs facing towards the two rooms (Sector A and C), and give them a Line Type of 1 (DR Door Open Wait Close). Make sure that your door's linedefs are facing outwards towards the two rooms. If you do not do this, there is a good chance the door will not function.
You should now have a door that, when being triggered, raises up to the ceiling, waits a few seconds, and lowers back down. But there is one more thing that should be done for aesthetic quality. Select the doorjamb linedefs on both sides of the door (highlighted in red), and give them the Lower-Unpegged flag. This will prevent the texture from moving up/down as the door is in motion.
There is an advanced technique that allows the mapper to make a door that does not open completely, but only halfway up, for example. To obtain this, first make a normal door and fully set it up the way you want. Once you have done that (I'll follow the example on the diagram), copy and paste sectors B and C somewhere outside the map. Transform the new sector C into sector D, but keep the copy of the door sector B (!). Now lower the sector D ceiling to the height where you want your door to stop, and that's about it, it is not necessary to make the copy of sector B a door as well. You're done. You've got a funny door that doesn't fully open.