Well, this one will get a bit longer, because lights are by far the most important things in doom: Every resource texture wad has different light-textures and -flats, so this is generally usable, no matter on what you are working on (also in Heretic maps or other non-futuristic worlds, you will find places where to add lights).
So, let's consider the fact, we are n00bs, our room is empty (although it has some crazy borders ;)) and we absolutely have no idea what to do with this. Then a simple idea comes to our mind: What about "LIGHTS"? So we create one sector in the middle of our room, alter the height so the light withdraws from the rest of the room and add a 255 brightness to this sector, finished! ... well ... to be honest, it isn't that easy to add lights to your room, as you will see now.

In shot 1 you can see one of the more simple but still nice-looking ceiling/floor light details. The border around this one is simply done by a heightdifference of 8, textured with the FLAT20 flat and the center of this is just a TLITE flat at the same height of the ceiling. These two textures can surely replaced with alternate versions. The result will look qualitative the same. To make it perfect, we just have to graduate the light from the center (between 200-250) to the outside (your rooms light level), this counts for every light design variation in this document! (buildtime 2-3 extra minutes, then copy&paste)

The second shot shows off another variant, but this time with two more borders and other texture variants (SILVER, SHAWN and this blue computer flat). In this shot you can also see how beautiful this looks on the floor (just to let you know, that we don't depend on using this on the ceiling). The height differences in this one lay between 4 and 2 units which makes this looking very flat. But this can also be variied. (buildtime 5-7 minutes, then copy & paste).

In the third shot we see a light very similar to the first and second screenshot but this time, the light-bulbs itself (including some TLITE floor material) have been stamped out of the ceiling, which adds 4 new sectors to your level statistics ;) (buildtime 4-5 extra minutes, then copy&paste)

Okay, most of the time, I just showed off lights from the ceiling, but don't forget, that we can do the same thing translated on the floor, why not? Shot four demonstrates floor lights/floor light platforms very well and they can also be used widely in every large outdoor or indoor area as you can see.

Finally in the fifth shot we take a look at a completely new variant of light-sources, the wall lights. Normally, just to achieve simple detail, we would add a LIGHT texture to the wall, build some small sectors around it (for the light gradient) and that's it. But in this case we want a high level of detail and that's why I have added (from the Nightmare Texture Packs) recolored LIGHT textures to the wall and placed 2 sloped TLITE variants in front of this, surrounded by a border as always (thx to the upper part). As a final touch, 3 round sectors with a light gradient have been placed in front of this wall light, floor/ceiling parts spotlighted by the planed TLITE's. I think the result is quite okay ;) (buildtime 6-8 extra minutes)

Another variant of a wall light can be seen in shot number six. Fredrik has used this effect the first time in Vrack 2 or even in Vrack 3 and when I created this light, I was directly inspired by his work. What you can see are several small sectors with a thickness of 1 unit, and each has its floor 8 units lower/ceiling 8 units higher then its parent sector. Also a light gradient has been added. The main textures used here are LIGHT and SILVER/SHAWN ones to get this high-tech space-light flair. It is an easy to achieve effect but a lot of work is connected with this. (buildtime 10-15 minutes)

In shot seven we will now get back to our ceiling-lights. In this screenshot, you see a almost floating light-construction in front of a skybox, which is just held by a few FLAT20 variant bars. Once again, you can see a border around the lights which make it look like a solid METAL object with a lot of electronical stuff in them to make the bulbs glow. (buildtime 10 minutes, then copy & paste)

In the last shot number eight we can see a variant of the light-source in shot six: This time, it just looks like a simple street-lantern and it is indeed, easy to do: Create one rectangular sector away from your building, add an 8-corner sector and within another TLITE thing, adjust heights, add border/lantern textures, adjust the light (gradient, as always) and finished is your simple but great looking - in a architectural sense - street-lantern, all belong to UAC :) (buildtime 5-8 minutesn, might take longer on complicated buildings)

Summary: Lights can be added everywhere and the good thing about it: You can reuse them as often as you want (just make prefabs or copy&paste them). The other interesting thing about them is, that they highly variabel, there is an infinite count of light-structures and everytime you start a new map, you start a new light-source. Just don't forget the following things: "borders, light-gradient, extrasectors for spotlights", then everyone will love you ;)
Extra-Buildtime: "2-10 minutes" (per light variant)
Build-Difficulty: "Low - High" (depending on light variant)