First, you need a
Zapthrottle Soul Inhaler, crafted by an Engineer, and some
Empty Soul Cages, crafted by a Jewelcrafter. These can be grabbed on the auction house.
Then go to one of these locations, and find the elemental you want:
When the elementals die, channel the
Zapthrottle Soul Inhaler item on them, and you'll get an Angry Caged Soul. Do not touch the item, and
leave it alone for 15 minutes. Then, you'll get a Docile Soul of the element you got and open that up to get the Elemental Soul item.