From the Super Mario Wiki
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After activating the Mini-Mario, the player has to move across a bridge with four Cannons placed under it. At the other end of the bridge is a Bob-omb dispenser and a blue Color Switch the player needs to press. With the blue Color Switch pressed, the player needs to return to the start of the level and use a blue ladder to reach the yellow Color Switch. With that Color Switch now pressed, the player has to use yellow platforms to reach a wire they need to climb across, leading the Mini-Mario across the bridge below. Once at the other side and the player drops, they simply need to jump across one more gap to reach the locked door.