In order to make an armourer villager, first you need to make a blasting furnace from five iron ingots, three smooth stones and a furnace using the crafting table. Take out these things from the chest box and put them in inventory, then place them in a crafting table to make a blast furnace. Add this blast furnace back in inventory.
Now you need to find a village to get to a villager. Find an unemployed villager and place the blast furnace next to him. He will become an armorer villager by doing this.
