How do I fill a black soul gem? [duplicate]

Ok, I have bought this game multiple times over the years on different platforms. The only real negative aspect of the game is turning into a vampire. I haven't played this game in about three years and just bought it on the PC for the first time. Somehow I've turned myself into a vampire, AGAIN! I didn't eat vampire dust, so I don't know what it is. My problem is getting rid of it. I've already gone too far in the stages.

Falion is my only hope, but I don't understand how to fill a black soul gem. I've grabbed the Spell Time: Soul Trap and used it, nothing happens. It really pisses me off since it keeps costing me money. Other forums weren't clear on this matter. Can someone please explain exactly how I can fill the black soul gem and cure myself? Please be specific.

A black soul gem can only be filled by the soul of a living human, or a creature with a Grand soul, such as a Mammoth (thanks to tjd for pointing this out!). If you'd rather go down the creature route, there's a discussion here about which ones have grand souls. First, aim at your target and cast the Soul Trap spell - it fires like an arrow (without the gravity) so plan ahead. Once the spell hits them they should turn a slight shade of purple, and you'll hear a sound to indicate the hit.

Now you have to kill them - I believe the effect lasts for around 60 seconds. If you go past this limit you'll have to recast the spell.

If you want to kill a human - which will be easier than the toughest creatures - you don't have to murder some innocent civilian - a bandit or warlock will do.

As long as the spell was still active and you have a black soul gem in your inventory you'll hear a sound and see a message telling you that the trap was successful. You will then have a filled black soul gem.

FYI you were probably in a fight with a vampire and contracted Sanguine Vampirisis - after three days that disease turns to vampirism.

Source: 5 and a half years of active Skyrim playing.