>>11895759
It's generally pretty easy. Doom level editors used to be clunky pieces of shit in the 90s, but then Doom Builder and better (built in) nodebuilders revolutionized Doom mapping in the 2000s, and most people are still using something like Ultimate Doom Builder now. It's a very easy to use program, and there's good guides for it.

For managing content in .wad files (like adding new music or textures), you would use Slade, which is also largely easy to use.