Load up the tutorial world. Once you're in, connect the second controller, press the menu button and enter as a guest (or any other account you have available on your console). Walk up the stairs and take the iron pickaxe out of the chest in the room with the Enchantment table. Once you have it, take out your map and walk towards X: 9, Z: -34.
Dig straight down until you find the diamonds. Once you've mined them, make your way back up by using the cobblestone you've collected. Then just throw the diamonds at the other player you created.

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