To find the game files for Doom and Doom II, you can navigate to C:\Program Files (x86)\Steam\steamapps\common\DOOMEternal\base\classicwads.

In the classicwads folder, copy and paste the DOOM.WAD and DOOM2.WAD to your desktop or another safe folder of your choice.

GZDoom is a community made source port that include modern features for Doom, such as widescreen and mouselook capabilities and modding support.

You can install GZDoom here: https://zdoom.org/downloads

This will install a .zip folder which you can extract in either the desktop or another directory of your choice. Once you have GZDoom installed and unzipped, you can drag DOOM.WAD and DOOM2.WAD into the folder. You should then be able to run the GZDoom application and play the classic Doom games!

With GZDoom, modding the classic Doom games is very simple. When you download a mod, it will either be a .wad file or a .pk3 file. If it's one of those file types, you can drag the file on top of the GZDoom application in the file explorer and GZDoom will run Doom with that mod.

Some recommended places you can visit to find mods are:
- ModDB (https://www.moddb.com/games/doom-ii)
- DoomWorld (https://www.doomworld.com/)