Celebrating the anniversary of a game well into its double figures is often a retrospective; looking back at a long-gone relic that has influenced gaming in one way or another. But what’s curious about id Software’s grungy first-person shooter Doom, which turns 25 this week, is that it’s still very much alive, ever-burning as relentlessly as hell itself.Apart from its gratifying gunplay, the reason the original Doom has survived this long is the modding community, which has been twiddling with the Doom source code ever since it was made publicly available in 1997. From cyberpunk dystopias to super-modern, ultra-gory overhauls, we’ve listed our favourite Doom mods here. But to get in on the carnage, you’ll need to know how to mod Doom, so we’ve created this guide to give you all the tools you need to play Doom mods, combine mods together, and supercharge the game for modern machines.Note that you’ll need to own Doom to mod it. In the install directory, each of the original Doom games has a WAD file – an archive file containing vital game data like monsters, sounds, levels and textures – which is required for the mods to work.Which open-source Doom port to useIf you’ve…