First, you need a
Inhalador de almas de zapacelerador, crafted by an Engineer, and some
Prisión de almas vacías, 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
Inhalador de almas de zapacelerador 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.