From the first blood-floored area, head left and fall into the large bloody pit. After getting the yellow skull key, continue up the stairs to the river of blood where you can run across to the teleporter floating in the large blood pool. This takes you to a platform high above this area. From this ledge, you will be able to drop down onto a lower one just south of the one you stand on. The teleporter close by takes you to an otherwise unreachable ledge leading to a soul sphere.
Drop down from secret #1 and head through the opening to the right, falling into another large blood pit area. Continue into the alcove to the left and ride the lift up to the ledge. Straight ahead of here is a gargoyle-faced switch. Press it, then continue up the nearby stairwell. At the top is a teleporter to a secret area.
It is possible to finish the map with a rocket jump combined with an Arch-vile jump. The speciality of this trick is that you will die during the performance, but you can land exactly on the exit what is enough for the accomplishment of the map.