Select a level with a white Toad marker. Listen for Toad when the level starts then break the "?" block that he is inside to free him. Reach the end of the level to make a mushroom house appear. The following types of mushroom house with a mini-game or reward will appear, depending on the last number of the timer when you collect the flag to end the level.

Green house with 1-Up Cannon mini-game: Last number of the timer is 0, 1, 2 or 3.
Red house with Match mini-game: Last number of the timer is 4, 5 or 6.
Gold house with Star reward: Last number of the timer is 7, 8 or 9.

Successfully complete the game to gave all Mushroom Houses in Worlds 1 through 8 reappear.

To have Mushroom Houses to appear in multi-player mode, complete the level with the last two numbers of the timer have the same digit, except for zero. The original Super Mario Bros. level completion music and fireworks will confirm that it has worked.
Gold Mushroom House: Last two numbers of the timer is 99.
Green Mushroom House: Last two numbers of the timer is 11 or 22.
Red Mushroom House: Last two numbers of the timer is 33, 44, 55, 66, 77, or 88.