This release of Instant Doom is bundled with Chocolate Doom and DOOM1.WAD (Shareware Doom 1)
To start playing, extract the ZIP and open Instant Doom, then click Launch under The Ultimate DOOM or hit the 1 key on your keyboard.
Chocolate Doom not your thing? Move or copy the Instant Doom EXE file somewhere your source port resides and enter the path for it in the source port settings (refer to README), then start the game the same way.