Also, you'll need a legit copy of your DOOM2.WAD file, either from a copy of your own discs, or from a digital distributor (I purchased & downloaded my copy of Doom II through GoG).
Add D44D, GZDoom, any other resource packs you've downloaded & your WAD to the same directory, double click D4D DragAndDrop to start the launcher, and you should be good to go! Have a blast!