Find a villager without a profession
Place the correct job site block near the villager
Wait for the villager to claim the block
Make sure the villager can path to the block
Do not trade with the villager if you want to change its job later
Break and replace the job site block if needed
Use the matching block for the profession you want
Lectern for librarian
Composter for farmer
Smithing table for armorer
Blast furnace for weaponsmith
Grindstone for toolsmith
Cartography table for cartographer
Fletching table for fletcher
Brewing stand for cleric
Cauldron for leatherworker
Barrel for fisherman
Stonecutter for mason
Loom for shepherd
Smoker for butcher
Bell for no job assignment in some versions
Ensure the villager is not a nitwit
Keep the villager in a safe area so it can work and restock