1) Make sure that Viktor Novikov goes out on stage in Sebastian Sato's place.
Either start as a make-up artist or sneak into the make-up area using one of the disguises (make-up artist, security, bodyguard, Helmut Krueger).
Turn on the diorama by using a coin. If you didn't take the coins during the preparation stage, there is one conveniently lying on the ground near the diorama.
Follow him to the secluded area, knock him out and hid his body in the chest nearby.
2) Get to the 2nd floor (the easiest way is to use the stairs to the left of the big staircase at the entrance, knock out the lone stage crew member at the top of the stairs and use his disguise).
There is a room with only one palace staff member. Close all the doors to be sure you are not seen, knock him out and take his disguise.
If you are carrying a firearm, be sure to hide it somewhere.
3) Get to the 3rd floor (you will get frisked by the CICADA - that's why you have to hide your gun beforehand).
Enter the door on the left (there is a staff member in front of the door).
Vault the first window to get out on top of the light rig.
4) The next part is a bit tricky.
You have to get a crowbar. Conveniently, there is one beside the window on the other side of the light rig. The problem is there are two bodyguards surveilling the area.
Wait until both bodyguards turn away from the window, get to the other side of the light rig, drop down, climb through the window and get the crowbar.
Wait until the coast is clear and return to the light rig.
Wait until Viktor goes out on the stage and use the crowbar to drop the lighting down.