After having lit the Bonfire you now need to kill the 4 Skeleton Conjurors that appear. They are very mobile with their teleportation and bolt spells. Explosive-type damage seems to work fairly well on them. Each time one of them is killed, they will drop a Soul that you can collect. When you have defeated all the Skeletons then you can collect all the souls at once. You can also do it as you destroy the skeletons optionally.
The Second time you need to defeat 4 Condemned Skeletons to collect the souls. Keep around mid-range to avoid the worst of what these foes can do. They can explode on you.
The third time you need to defeat 4 Fractured Skeleton. Blast away using explosive rounds or AOE attacks for some quick kills.