Saturday Morning Cartoon Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in New Mombasa Streets (preferably at the Campaign’s start). 

At the start of the game, proceed through the mission as normal until you have to activate the Superintendent. After this occurs, you’ll receive a waypoint on your HUD, instructing you to enter a nearby building in order to pass into another section of the city. Make your way through this building and before exiting the door that leads to a courtyard of sleeping Grunts, stop before door and check the right side of the room. You should see a small couch and an even smaller box of cereal on the ground towards the far end of couch (it’s not that hard to see). Walk up to the cereal box and stare it for a few seconds, then the achievement should unlock.