Not sure if anyone needs one, but I haven't seen a text guide posted for this so I'll go ahead and make one. I would recommend trying to figure it out for yourself, since most of the steps are pretty self explanatory if you listen to the quotes, but if you don't want to do that, then here are the steps:

Step One: Shoot the Soldiers
The first step is to shoot the four green toy soldiers scattered around the map with an M16. They are located in the following locations:

1. In spawn between the elevator and Quick Revive
   
2. On top of the wallbuy Tommygun location near Jug
   
3. On top of the Power Switch
   
4. On top of Pack-a-Punch

If this is done successfully, you will hear a quote talking about finding "the Shape in Pack-a-Punch".

Step Two: Shoot the Shape
For this step you need to teleport to Pack-a-Punch and shoot The Shape (otherwise known as Michael Myers), who is hiding behind the door downstairs with the CEL-3 Cauterizer on it. I'm not 100% on this, but I think you might also need the M16 for this step. If you've done it correctly you'll get a quote that congratulates you, and says there's a party favor for you, after which point you'll hear the sound of an elevator dinging.

Step Three: Shoot the Cupcake
Now you need to go back to spawn, where you will be able to enter the elevator and purchase it for 10 points. It will send you up to the security room from Five Nights at Freddy's, where you will need to shoot a cupcake that is sitting on top of one of the monitors. After this you'll get a quote that mentions you need to get kills by the teleporter.

Step Four: Kills by the Teleporter
This part is pretty straightforward; all you need to do is get some kills around the teleporter pad until you get a quote (I wasn't counting, but I'm gonna guesstimate it was around 30-50ish) that mentions finding a switch in the control room

Step Five: Shoot the Nintendo Switch
Now you need to return to the Five Nights at Freddy's room, and if you prone to the left side, you'll see a Nintendo Switch hiding in a vent. Shoot this, and you'll be rewarded with a Perkaholic and a quote teasing the next map.

Optional: End the Game
Now that you've completed the Easter Egg, feel free to buy the 93,500 ending in the elevator near the Power Switch, the door of which costs 10,000 to open.