Gelion
Retired Captain
Thats right.
Basically to soultrap you need to have a "soultrap" scroll or spell. Cast it on the cerature (may hurt as when you cast you are defenseless) and kill it. If you have a right soulgem you'd get a message "You have trapped a soul". More difficult creatures requre greater gems. Since you are a thief you should have plenty
.
There are mods that allow you to trap NPCs
. I did that with Fargoth! 
Basically to soultrap you need to have a "soultrap" scroll or spell. Cast it on the cerature (may hurt as when you cast you are defenseless) and kill it. If you have a right soulgem you'd get a message "You have trapped a soul". More difficult creatures requre greater gems. Since you are a thief you should have plenty
.There are mods that allow you to trap NPCs
. I did that with Fargoth! 
. The Umbra-sword has soultrap skill that isn't constant effect, but when you use it it lasts for 60 seconds
Let me test it out and see if i can summon a Golden Saint.


