Open a crafting table
Place 7 iron ingots in a cauldron shape:
Put 1 iron ingot in the top-left
Put 1 iron ingot in the top-right
Put 1 iron ingot in the middle-left
Put 1 iron ingot in the middle-right
Put 1 iron ingot in the bottom-left
Put 1 iron ingot in the bottom-middle
Put 1 iron ingot in the bottom-right
Leave the center empty
Take the cauldron from the result slot