1. download and install zandronum
https://zandronum.com/download

2. download the iwad doom2.wad and put it in the same folder as your zandronum install, wherever zandronum.exe is located
http://a.pomf.cat/zzofqy.zip

3. launch zandronum.exe, point it at your doom2.wad if it doesnt find it automatically, start it in HARDWARE mode, and configure your settings, many default keybinds are quite old school so you may consider a total remake of the controls (unless they changed this since I downloaded it). Go to mouse options and turn always mouse look on, and lookspring and lookstrafe off. go to player options to set your name. go to hud options, message options, then text scaling, turn it up and test it out to what you like, the default chat is incredibly tiny. go to display options, opengl options, texture options, texture filter mode to none (this is a personal preference but the filter sucks). adjust the rest of the settings as you want and experiment to your liking

4. start doomseeker, make sure under games>zandronum that it points to your zandronum.exe. under file paths, make sure it points to your base zandronum folder, and it is highly recommended you make a new folder in an easily accessible place called "wads" and set another file path there. under wadseeker>general set the path to your newly created wads folder, otherwise it will place all automatically downloaded into appdata, and thats no fun

after that it should be all set up and you'll never have to fiddle with any major settings again, you're now ready to drop wads in your folder and join any server and it'll all just work, probably