How do you trap a soul on The Elder Scrolls 4 Oblivion

You must have two things in order to be able to trap a soul.

1 - Soul Trap spell. This could be the spell itself, or an enchanted item. The item could be one with Soul Trap as a castable power, or it could be a weapon that casts Soul Trap on whatever it hits. The target must then be killed before the spell wears off.

2 - An appropriate Soul Gem. Soul gems come in different qualities, some creatures have a 'larger' soul and require a higher quality gem, for example an Ogre requires a Greater Soul Stone, a Savage Ogre requires a Grand Soul Stone. The game defines the sentient, intelligent races as having a 'black' soul, while beasts, daedra, unintelligent races like goblins, as having a 'white' soul. Only Black Soul Gems (and Azura's Star) can contain the soul of a human, elf, orc, khajiit or argonian, also the sentient daedra, the Dremora, and the Shivering Isles NPC races Aureals and Mazken count too.