/ws -create a soul gem (you must be holding a weapon with a soul tracker)
/splitsoul -split a soul gem
Soul trackers harvest the souls of defeated players. Every kill you get with a weapon that has one one adds a soul. Soul trackers are applied to weapons in the same manner as the custom books. A more detailed explanation can be found here.
Soul enchants can only be obtained from legendary envoy chests. They are applied just like any other enchant, but require an active soul gem to function. These enchants can be divided into two groups; active and passive. Active soul enchants drain souls from your soul gem every time they proc. Passive ones slowly drain souls at a fixed rate.
Soul gems are created by withdrawing souls from a weapon. The quantity of souls you withdraw are given a multiplier based on the tier of your soul tracker.
To activate a soul gem, right click it. A more in-depth guide can be found here.