This is a tutorial on making mods for Doom or Doom 2. This tutorial also works for editing Heretic or Hexen (I think.)
PART 1: SPRITE TUTORIAL
Tools needed for modding:
1: XWE - eXtendable Wad Editor
2: Doom Builder
4. zDoom (best for loading mods)
First Tutorial: Adding Sprites...
Okay, you've played it, now you want to make it. Now doing this won't be extremely easy, you have to work. First, open up a new version of XWE if you have it. Go to the section at the bottom called SPRITES. Sprites are anything that moves, shoots, is able to be picked up - basically anything that isn't the walls, ceilings, skies, floors, coding, or surfaces. The first sprite that normally comes up is CHGGA0, the "chaingun barrel rotation sequence 1" sprite. If you want to change it or any sprite you want to (let's just do CHGGA0), go up to the top and select ENTRY, REPLACE (not Replace RAW DATA) open a folder, choose an image, with a light blue or purple background. If it has one of the two color backgrounds, then it can be successfully converted to a sprite.
Now, three things can happen here. The first thing that could happen is that the weapon is perfectly settled on the HUD simulator. The second thing that could happen is that the sprite could be NOT on the HUD simulator. This means that you didn't take that sprite from another WAD. It hasn't been configured yet. The third thing is that the weapon is chopped up, super large, and awful-looking. The sprite is too large. Go ahead and resize it down a bit.
If you want your sprite to show up on the "HUD-style fix," which you normally will if your sprites aren't too large, then put a "-" (without the quotes) before the coordinate's number. By the way, if your sprite is too big, it will not shrink it down.