How to ignite / fire build? (Assassin's Creed: Valhalla)

Actually, it is not clear how to make a fire build, like in the Odyssey, so that the weapon sets fire to enemies.
There are donate sets for bonuses to this, there are runes for fire damage too, but how to set fire, except how to poke a torch, is still unclear.
can that where to find and study it is necessary?