Trapping souls

[Fishlore]

I'm a mage, how do I trap souls in soul gems? I have some soul gems. There empty, best I can tell. I've seen tips during loading mentioning you can trap the souls of creatures you conjure. I'm just missing the connection between the two. Anyone know how this works? Thanks.

 

[Glirk Dient]

Cast the trap soul spell on them and kill them while its still active.