At the very beginning, make sure that the guys that stand in your way will be A.R.T. (Assuming Room Temperature). At the edge, before you jump down on the... guts below, look right. There is a eye-on-a-stick switch. When you activate it, a concealed door open behind you with a goodie.

Jump inside the concealed room from #1 and tap the wall to the right. Here is #2.

Get across the gut floor, get the lift up. Look to the right. Walk on the narrow ledge around the torch to get the next secret.

Back again? Get up and grab the red key. At the side of the platform opposite the key, look down: a teleporter. It will take you to the next secret.

...Just a little useful hint. Sooner or later, a baddie will arive at the entrance. In order to save a considerable amount of ammo, get back on the gut floor, and as close as possible to the edge. When Mr. Baddie is in the right spot, use one of the concealed teleporters and telefrag him. However, if he isn't in the right spot...