[B_part]

To trap a soul you must cast the spell upon a living creature, then kill it before the spell expires.
Obviously you need a soul gem, and it must be powerful enough to hold the soul (e.g. a Golden Saint will be held only by a Grand Soul Gem, while a rat can be put in a Petty one.)
Also, if i recall correctly the same soul will perform differently in different soul gems: the better the gem, the better the effect.

The spell (Soul Trap) is a Touch spell, which means that you must touch your enemy while casting it in order for it to be effective. If correctly casted, you will see the creature surrounded by a pink fog ring.
It is governed by the mysticism skill, which means that if you are terrible at mysticism, you will fail often (and trust me, failing while a Golden Saint is spanking you is usually a bad idea).
It doesn't work on non-player characters (e.g. you can't soul trap anybody which has a proper name e.g. no Sjoring Hard Heart or Sugar Lips Habasi but works on Ascended Sleeper)

Three solutions to the problem:

1)improve your skill (via training in the mages guilds) I strongly reccomend this option, since Mark, Recall, Almsivi and Divine Intervention are governed by mysticism as well, and being able to cast them helps you with your wandering.

2)buy a scroll (Ephiggy's Gem Feeder - Galbedir in the Balmora Mage's guild has them; I guess all enchanters have them). Using a scroll you can't fail, but scrolls cost (120 drakes If I Recall Correctly)

3)Enchant an item or find one already enchanted (possibly a weapon). In the game you should find some.