First you need to download GZDoom first of all, its not that hard, just download GZDoom for your operating system here: https://zdoom.org/downloads

Extract the contents of the zip folder into your DOOM or DOOM II directory in
Steamapps > Common > The Ultimate DOOM > rerelease
Steamapps > Common > DOOM II > rerelease
and extract the stuff in there. now, rename the original DOOM executable to something different and name GZDoom.exe to DOOM or DOOM II.exe, now launch DOOM or DOOM II and you will get greeted with the GZDoom launcher, select the IWAD of choice, DOOM or DOOM II and run it.

Once in GZDoom, go to video settings, you might have to go to full options menu to find it and change the Render mode to one of these 3 Modes and either turn these settings off in the mouse options as well.
True Color Software - No Mouselook / Autoaim - Doom Software but with more colors.
Doom Software - No Mouselook / Autoaim - Original 256 Color look.
Hardware - Good Mouselook - OpenGL Rendering with different lighting.


Texture Filtering!
I highly recommend turning Texture Filtering Off to get that original look, go to display options > texture options and turn all the settings to there off state.

MOUSELOOK / AUTOAIM

Autoaim > Player setup and slide the bar that's labeled autoaim
Mouselook > Mouse options and find "ALways Mouselook and turn it on or off, I dont recommend Mouselook, mainly because you aren't meant to look up or down, its just my preference, you don't have to do it but i recommend it. and if you do use Mouselook. I only recommend it with the Hardware render mode, because the other 2 are sort of warped because it uses the original doom engine without hardware acceleration.

The only thing you really have to configure now is controls, but that's preference, have fun and start blasting Demons!!!!