With advanced Doom ports like Boom and PrBoom+, modifying Doom is as simple as packing a text lump named "dehacked.lmp" in the WAD. When using DeuTeX to build the WAD, make sure "dehacked.lmp" is in a folder named "lumps" and make sure "dehacked" is in your wadinfo file like so.
[lumps] dehacked
The DeHackEd lump is a simple text file with blocks that serve to define and modify Things, Weapons, Frames, and more.