This tutorial is relatively simple, I am not very good at modifying a game's files and I had no problem with this. Just and FYI this works on all of the other classic Doom games EXCEPT Master Levels for DOOM II

Go to the GZDoom Website[zdoom.org] and download the appropriate file (most will be downloading the Windows 64-bit one) and put the file somewhere you will remember for now

Right click your Doom game and select 'Properties'. Go to the local files tab and press 'browse local files, you should end up in the location of your Doom game

Drag and drop all of files from GZDoom into the 'Base' folder

Delete the dosbox.exe file and rename the 'GZDoom' executable to 'dosbox', this is CASE SENSITIVE so spell it right!

If you did everything right, you should be able to boot up Doom through steam, if nothing works, double-check the previous steps

Time to tweak the settings! This depends on how you like the game. To make the game feel modern, I recommend having alwaysrun on and turn on always mouselook so you can look up and down. Unless you like classic Doom controls you will also need to change the movement bindings

Thanks so much for taking the time to read my guide! Let me know if you are having any problems in the comment section. I plan to make more guides to make classic games look new in the future so make sure to drop a suggestion.