The episodic format is not unlike the one in the first Doom game, known since 1995 as The Ultimate Doom. Each episode will have eight normal levels, and one secret level. Because of the advancements the ZDoom source port gives mapping wise, I can guarantee you finding the secret exits will be a daunting task! You can do it!
I will give you a hint, in the map "The Cold Shoulder," you will be required to do a strafe-run jump in order to land on a platform to trigger the secret exit, I highly recommend quick saving though, for in the area it's at, you can fall to your death, and you will not want to have to reload somewhere saved at an earlier time and have to redo everything you did!