From the Super Mario Wiki
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The player starts next to a blue Color Switch, which they need to press to have the Mini-Mario above fall onto them. They then need to jump up a series of platforms, using Falling Spikes as additional steps. Next, the player has to time jumping onto a red Color Switch to turn the red Polterguys below into platforms, which the player has to cross to reach a couple of Springboards. After the Springboards is a spike pit with two blue Polterguys floating above it and a blue Color Switch. Timing the press of the Color Switch with the Polterguys' positions will allow the player and the Mini-Mario to reach the locked door.