Have it sitting on your hotbar as you gain XP like any other skill in the game. This is not Skyrim. You do not even have to use it. Just needs to be sitting on your hotbar.
The active hotbar. the secondary bar will not gain XP.
If you are talking about the skill, it will level as you gain xp if it is on you active hot bar. If you mean the skill line, that ranks up only by completing the main quest. I believe you should have all passives unlocked by the time you reach Coldharbour.
Buy a ton of empty soul gems start using soul trap and filling them up its the only way, Ya gotta actually fill souls gems for it to count and it feels like it takes foreverrrrrr
The last passive is part of the "Soul Magic" skill line. You can level the individual skill called Soul Trap by using it, and then morph it, etc. But the passive skills in the Soul Magic line get unlocked slowly but surely as you advance through the Main Story to its conclusion.