Firstly, make a Crafting Table using Wooden Planks if you don't already have one.
Next, use the table to make the iron armour, you'll need 24 Iron Ingots. You need to craft an iron helmet (5 ingots), chestplate (8 ingots), leggings (7 ingots) and boots (4 ingots).
Finally, open your inventory (
) then drag the pieces of armour into place. You should now have the achievement.
Hope this helps!
EDIT: According to comments, you now also have to craft a shield for the achievement. https://minecraft.fandom.com/wiki/Shield