From the Super Mario Wiki
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The player starts next to the Mini-Mario. After activating it, the player has to lead it across a bridge with two Lava Bubbles jumping up from below. Once across, they have to jump across a small platform with a 1-Up Mushroom on it that also has a Lava Bubble jumping up from below, then use a Springboard to reach an upper platform. Using another small platform the third Lava Bubble can reach, the player can jump to a low hanging block they can use the Mini-Mario or a Handstand Jump to get over, followed by another long platform to cross that the first two Lava Bubbles can jump over. At the end of the bridge is the locked door.