From the Super Mario Wiki
The player must first enter the pyramid and climb to the top, as if they were doing the course's third mission, Inside the Ancient Pyramid. Another way to do this is by standing on the four pillars to activate the secret entrance, as in the course's fourth mission, Stand Tall on the Four Pillars. Once the player reaches the top of the "sandfall" just before the Power Star, they need to climb down to the small ledge below with a Coin. Collecting the Coin will start a secret counter and collecting five of the special coins in the pyramid is the mission's objective. The player then has to jump down to a small platform on the left side of the sandfall, followed by jumping to the platform on the right side, collecting the Coins as they go down. The player must then jump into the stream of sand and collect the two Coins within it. If the player collected all five coins, the Power Star will appear at the end of the stream. However, the player has to be careful when collecting it, otherwise they will fall into the quicksand below.
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