From the Super Mario Wiki
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The player has to jump up across several sand platforms positioned over spikes using Backflips or Handstand Jumps in order to reach the platform with the Mini-Mario on the upper-left. Once the Mini-Mario is activated, the player has to climb down a couple of ladders and grab the Barrel at the bottom. They then have to move past the stair platforms on the left, and place the barrel so the Mini-Mario and the player can use it as a platform to get back over the stairs. The player can then unlock the door to the next level.