Here is a quick and easy guide on how to get this ending. (SPOILERS FOR THE ENDING!)

1) Play though the game normal till you reach the elevator in the boss's office.
2) Go down the elevator.
3) Go back up the elevator.
4) Enter the boss's office again.
5) Go back down the elevator then go back up.
6) Re enter the boss's office and wait till the narrator has enough time to sink in the realisation that the boss's office is still there. (You will know he has had enough time after 2 "Please hold" screens have gone past.
7) Go down the elevator.
8) Go back up the elevator, but this time however it will slow down after the loading zone and the narrator will start a long speech explaining how Stanley is good at creating a story.
9) The elevator will split you out in a white hallway, follow the carpet till you reach a set of metal stairs.
10) At the top of the stairs is a metal door, one you go to it the door will open into a large stadium of people.
11) Stanley will soon pass out and you will restart the game.

Congratulations you have gotten the Ending.

Bucket differences
1) Instead of simply entering the boss's office you will need to go over to the keypad and press the number 3 every time you go up the elevator
2) Once reaching the press conference section it will be revealed that no one has come and then the game will restart