Get an empty soul gem
Learn or use a Soul Trap spell, weapon enchantment, or bound weapon with Soul Trap
Cast Soul Trap on a living target before it dies
Kill the target while the Soul Trap effect is active
Make sure the target is not a humanoid with a black soul gem requirement unless you are using a black soul gem
Use a soul gem that matches the soul size of the target
Check your inventory to confirm the soul gem is filled
Use Azura’s Star or the Black Star if you want reusable soul gems