Instructions
Win the Super Smash Brothers game eight times, using one of the original eight characters each time. The Nintendo 64 version of "Super Smash Brothers" comes with eight playable characters: Link, Mario, Kirby, Samus, Pikachu, Donkey Kong, Yoshi and Fox. There are four secret characters that can be unlocked during the course of the game (Luigi, Captain Falcon, Ness and Jigglypuff), but you do not have to beat the game while playing these characters in order to unlock the Mushroom Kingdom secret stage.
Defeat the game using all eight players and compete on all of the eight battle stages. This is a useful technique for those who have won the game in one-player mode using all of the original eight Super Smash Brothers players, yet find that they do not have access to the Mushroom Kingdom secret stage. While all game walkthroughs for Super Smash Brothers state that using all eight players is necessary for unlocking the Mushroom Kingdom secret level, there is debate as to whether or not playing on each separate stage is necessary as well. Play each stage with each character at least once to safeguard your victory.
Complete the game as described in Step 1 and Step 2. Create a new game, then go to the "Stage Select" screen after selecting your player mode (story, one player, versus) and choosing your character. The list of stages should now include nine different environments, including one that resembles the original Nintendo classic "Super Mario Brothers." This is the Mushroom Kingdom secret level. Click on it to select and play.