Soul trapping

I have the soul trap spell, but I'm trying to figure out how it works. Do I cast it then switch to my weapon or do I cast it and hope my follower kills the baddie. I can't figure that one out. Im building up my enchanting skills so that I can enchanting a weapon but want to have more than 4 seconds for the baddies to die for it to work. Any tips and tricks would be helpful. Thx