ULTRAKILL: Layer 2 Secret Level Guide

Starting off you want to select The level 2-3: Sheer Heart Attack

Proceed through the level as normal until you reach this room with a waterfall in it.
After killing EVERYTHING look under the waterfall’s bridge to find a crawlspace.
Slide into it to reveal the first switch, make sure to shoot/punch/blow the electrical box up.

After using the blue skull from the fountain room you’ll reach this combat area.
As soon as you enter the room turn left and look up to reveal a vent in the corner of the room.
Wall jump up to the vent to reveal the second electrical box. Again shoot/punch/blow it up.

After clearing the previous room proceed through the level as you would and acquire the red skull.
MURDER the Mindflayer and open the door with the red skull pedestal.
You don’t have to place the skull on the pedestal, instead turn around and check under the stairs to find the third and final electrical box, you know the drill shoot/punch/blow it up.

After getting all 3 electrical boxes (the order in which you break them doesn’t matter.)
You will receive a message confirming that you unlocked it.
Backtrack to the waterfall area where you found the first electrical box to reveal it is no longer running.
Go inside and drop down to enjoy the new secret level.
Hope this helped!