What do you Need:
. 3D glasses
. An empty/cleared Slot
. A trainer or Cheat Engine for FEZ

Which are the steps?
1. First of all, you need to get a trainer for FEZ (check the Game.Copy.World.Site)
2. When you get it, start a new game.
3. Climb up and get the fez (do not touch any Bit cube yet, just climb up)
4. Now after the “game restart” inside the party, go outside of Gomez bedroom and activate the “Infinite Cubes” Cheat.
5. Now Go down and activate the violet/blue door.
6. Try to avoid the Bits Cubes here by activating the “Super Jump” Cheat and Go Up to the 4 Doors Room.
7. Once inside, Tap LT just once (or A if you use keyboard) and get inside.
8. Tap LT (or A) Twice, and climb up to the violet/blue door, activate it and get inside.
9. Now before you enter to the final world, deactivate or quit the trainer first (to avoid any bug).
10. Finish the game and you will get the Sunglasses and the 3D glasses at once.
11. Contratulations! Now you can complete the whole game in 3D Stereoscopy Mode! ;)

If you couldnt follow any of these steps, I will leave here my savegame if you want to see it:

http://www.mediafire.com/download/k33cmvo9yc9efjd/Fez+BackUp+04-12-14.zip

Only you have to do is download it and copy the files to %appdata% /Roaming/FEZ folder.

I hope that this has helped you and sorry for my bad english.
Thanks for reading guys!! ;)