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
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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

