After entering the first hallway in the level (accessed through the southwest door in the room after the one where you begin the level,) head south and open the eastern door to find a zombieman and a box of shells. (sector 247)
Open the door southwest of secret #1 to find a few enemies and a medikit. (sector 166)
After dropping into the passage leading to the elevator which takes you to the yellow door, go west, head through the next door, and you will find a room with multiple monsters, a box of shells, and eight health bonuses. Open the door in the southwest of the room to find another room with five health bonuses, a box of bullets and an energy cell pack. (sector 238)
From secret #3, head upstairs to find another room with a zombieman and (on Ultra-Violence or higher) a lost soul. (sector 233)
Go up the stairs from secret #4 to encounter a zombieman. (sector 225) On Hurt Me Plenty or higher, an imp will accompany him; and on Ultra-Violence or higher, there will be a second zombieman in the room.
At secret #5, go up the steps to find two zombiemen, a shotgun guy, and two health bonuses. (sector 218) From there, go up the next staircase and open the door to return to the start of the level.