Doom modding

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I want to mod doom, but I haven't a clue where to start. I've done many googles searches, from "how to mod doom" to "doom modding tutorial" but I always get databases of doom mods, or tutorials on how to mod doom 3. Help...    ._.

[maia]

Planet Doom or Doom Planet. There is also Doom Nation. There are quite a few massive DOOM websites that explain everything :-) I would always go to them, but it has been 4 months so I cannot remember the names of them. doomworld.com planetdoom.com doomplanet.com?