This submod adds the Durak card game to Monika After Story. If you haven't heard about it, Monika can explain it to you, or you can find out here. It unlocks sometime between NOU and piano. If you've already unlocked piano, Monika should bring up the conversation which unlocks the game a few minutes after launching MAS with the submod installed.
Download the latest version zip. It contains the Durak folder which you should put into your Submods folder.