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.