How do I trap a soul in a soulgem?

Cast soultrap and then kill. It will then say, you have successfully trapped a soul. Of course, make sure you have soul gems in your inventory.

I had the hardest time understanding this. At times, when cast, magical energy will encircle the beast (soultrap). I tried it on a Knix Hound and did not see the energy around the beast when I cast it, so I did it like four more times to make sure. It does not matter, just cast the spell once and make certain to kill the beast before the spell's duration wears off. Also, if you want to trap a certain beast in a certain gem, make sure you drop all gems of lower value (as in you wanted a common soul gem, you should drop all petty and ¿uncommon?)

OK thanks. I just want to know how I can get a spell trap.

your making the newguy really think hard about this soul gem stuff arent ya.

all you got to do is kill a nonhumanoid creature. yes just kill it.
then cast soultrap on it. it does capture the soul after its dead.
and for me it already puts it in the highest soul gem it can go in