How do you capture souls?

Try casting a longer lasting spell,make sure you have an empty soul gem and make sure it's big enough.

You either need a capture souls scroll or an enchanted weapon such as Umbra or a spell which captures souls you will also need soul gems the size of the soul you can capture depends on the size of the gem that you are carrying in your inventory with the Umbra sword all you do is attack them and it will automatically capture the soul into an empty soul gem if you have one big enough with the scrolls and spells you need to equip them and use them like your magic spells and it will capture the souls into an empty soul gem Umbra is a lot easier though

You must buy a soultrap spell or use Umbra. The soultrap spell is in the mysticism school. Umbra is a special sword. You must use soultrap (or Umbra) and kill the NPC within the duration of the spell. You must also have an empty soul gem in you inventory that is large enough for the soul you are trying to catch. I suggest looking in the Arcane University for grand sould gems. Also, you can do the shrine of Azura and get Azura's star which is a reusable soul gem. And that is how you capture souls.