How does it actually work?

Do you cast it on them then kill them and it collects the soul or cast it before they die?

You must cast it before they die. Then, if they die within a ceratin amount of time (don't remember exactly, some seconds), you'll fill a soulgem (if you have one big enough in your inventory, of course.

There are also weapons with a soultrap-spell, I find them more useful on warrior characters (you can salso give them and a few soulgems to your follower).

I'm using bound weapons. Yeah I knew about the Soul trap for weapons. I'm trying to use nothing but Magicka. That's why I was wondering about the tome. I've been mostly getting grand gems, now that I'm stocked up on them, I'm going to go kill some Mammoths to fill them, But i was stuck on getting the souls considering im using Bound weapons. But Tyvm for the reply!!!