Find or create a village with an unemployed villager
Place a blast furnace near the unemployed villager
Wait for the villager to claim the blast furnace as a workstation
Make sure the villager has access to the blast furnace during work hours
Trade with the villager to lock in the Armorer profession
If needed, break and replace the blast furnace until the villager becomes an Armorer
If the villager is already linked to another workstation, remove that workstation first
If the villager is a nitwit or baby villager, use a different villager