First, you need a
Inhalateur d’âme Zappeur-Gazier, crafted by an Engineer, and some
Cage d’âme vides, 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
Inhalateur d’âme Zappeur-Gazier 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.