Craft an anvil by making an iron block and using iron ingots
Gather materials: 31 iron ingots total
Smelt iron ore into iron ingots
Craft 3 iron blocks (each iron block = 9 iron ingots)
Craft 1 additional iron ingot
Build the anvil in the crafting grid:
Top row: iron block, iron block, iron block
Middle row: iron block, iron block, iron block
Bottom row: iron ingot, iron ingot, iron ingot
If you want to use the smithing table instead:
Place an anvil in the smithing table and apply repairs/changes using materials (only if you already have an anvil)