1. Spend some time reading the tutorials. You will learn the fundamentals, and, most importantly, the terminology. The two tutorials that helped me the most when I was getting started was The Unofficial Wad Designer's Handbook by Ron Allen and Bill McClendon.
2. Open up an editor and begin exploring. After you've tried out a few editors and settled on the one with which you are most comfortable, open up maps that you've played. The original DooM and DooM2 maps are great for starters, as they contain examples of most things you can do with the original DooM engine.
3. Start making your own maps. Experiment with rooms, doors, windows, wall recesses, overhangs, platforms, stairs, and lifts. These type of constructs typically form the bulk of most maps.
4. Experiment with wall textures and floor/ceiling flats. You'll find that if you use constructs and textures within a general theme (e.g., military base, medieval, hellish) your maps will look more natural.
5. Experiment with the various effects -- teleportation, flickering lights, hazardous areas, crushing ceilings, etc.
6. Experiment with enemies, traps, and gameplay. Use enemies selectively, as each has strengths and weaknesses. Not every enemy works in every situation. Make sure that you give your enemies enough room to move and maneuver. Putting a "wide" enemy in a narrow corridor causes them to be stuck to the walls, defenseless and unable to move or attack.
7. Experiment with weapons, ammo, health, armor, and other powerups. Strive for a balance, so that the player is challenged but not frustrated. Give the player just enough to be able to overcome the obstacles you put in his or her way.
8. Work on the details of your maps. Pay attention to texture alignment, add architectural accents, add decoration and scenery.
Rex Claussen (Rex's Nexus)