I have soul trap as a Spell Tome but can't figure out how to use it. I can't make it a Favourite, and when I click on Use, it says I "already know it". I just shot an elk with my bow which killed it instantly but I didn't get the soul. Help?

If it says you already know it, then you have the spell. Just equip the spell and cast it at an enemy at close range. The spell and enchantment are different. The spell is used with your hands, and the enchantment (once you find and disenchant it) is placed on a weapon at an Enchanting Table.

You have to cast the spell before the NPC dies, then kill the NPC while the spell effect is active.

As for casting a spell, the Tome is just to teach you how to use the spell. You ready it from the Power Menu and choose the spell itself. The Spell is what you would also Hotkey.

Me, I use a weapon that has the spell effect, it makes this much easier

In addition to what Shadow said, you must also have the proper empty soul gem to capture the soul.

http://www.uesp.net/wiki/Skyrim:Soul_Trap_(effect)

Great. Thanks so much. Not sure why I find some of these things difficult except maybe that I'm ancient.

Thats the great thing about this forum. Everyone looks out for each other and helps out in any way we can.

I agree; you're all very good to take time to answer basic questions. Good people. I've experienced it on other forums as well. If only the rest of the world felt the same way about life...

Well you're going to get some "know it alls" or "spammers" or "trolls" if you will. But the majority of the people here are always willing to help and offer their experience and expertise. And to think I've been playing this game simce Jan '12 and still ask questions and get a ton of good advise from the people here.