That's actually not at all complicated.
But essences are confusing...
((Ok, so, let me explain how essences work.
During any turn (which is not a combat turn, I'm pretty sure that's a rule) you can create essences as an action. Creating an essence is basically casting a spell, albeit one with no real effect. This makes essence creation much less risky than spells which can backfire and have you immolate yourself by accident, as the worst that happens when an essence backfires is that some random event related to your aspect happens nearby. It is for this reason that most people create multiple essences in one turn, despite the fact that one essence = one spell cast, and multiple spells per turn carries a penalty: -1 cmp roll and -1 pot roll for each spell previously cast.
Now that the mechanics are out of the way, here's the point: an essence is a non-physical pattern of magic in your thoughts; basically a pre-charged spell which has yet to be given an effect and then cast. Maybe that's a bad analogy. Anyway, when you're casting a spell, you can choose to use essences from your essence inventory (basically infinite space since essences are not physical; visible in your character's block on the charsheet) and their bonuses will be added to the rolls made when determining how well your spell went. In the case of essences which have multiple bonuses, the listed bonuses are ordered competency first, potency second. Warning, though: essences take time to infuse into your spell, and each essence used will decrease dexterity rolls made to dodge by 0.5, and will decrease the speed roll to act first by 1. This absolutely stacks.
Oh, and, there's a limit of 5 essences per spell.
How an essence's power is determined, as my understanding goes, is what roll the essence creation spell succeeds with. For each spell, two dice are rolled: one for competency, which can only be enhanced by special means, and one for potency, which is a character stat. If the essence creation spell succeeds on competency it gains a cmp bonus, and the same goes for the potency roll. However, in the event of an overshot success (rolling a 6) something goes a bit wrong. Cmp overshots lead to pot penalties in the essence, and pot overshots turn the essence chaotic, meaning DG gets to screw with you when you cast a spell using the essence. (I.E. something random happens. DG does not actually screw with people all that much)
Now that I've gone over that in immaculate detail, the tl;dr version. When it's not a slugfest, you can cast a spell to make an essence. You can cast all the spells you want in a single turn, one essence per spell, but each extra spell has a -1/-1 which stacks. You can hold as many essences as you damn well want and pour up to five into one spell, which adds their bonuses to the spell's rolls. Each essence has a -0.5 to dodging and a -1 to going first when used in a slugfest. What bonuses an essence gets depends on how well you roll when you cast the spell to make it, but overshooting makes it get all funky. If the creation spell fails, your power seeps into the nearby area and does something.))
((Don't let the ol' wall of text fool you, it's actually pretty simple. If you've got any questions, ask away, I'm happy to help.))((Also, there are more creative ways to make a nondescript character. Here, let me copy-paste one of my backup characters' descriptions.))
Name: Jerremy
Affinity: signs (for example, wooden boards with letters on them. not (necessarily) metaphorical signs, as in omen.)
Description: A man who defies description. Every part of his body is mind-numbingly nondescript, so people usually end up describing him for his clothes, which are generically those of a road worker. The only descriptor that ever seems to come to mind when describing him is "sinister," but nobody's been able to say why such a scathing descriptor should be applied to such an average road worker as Jeremy, so nobody ever says it out loud.
STR=2
DEX=1
SPD=1
END=1
POT=3
Nobody ever paid that much attention to Jeremy; he was just that kind of guy. Likewise, nobody seemed to notice when he vanished to join the deathmatch between mages. People noticed something was missing, but never really tied it to Jeremy. Nobody knows why he left, because nobody really thought about it. Perhaps Jeremy wanted to strengthen his I-Am-Here signal? Maybe Jeremy wanted company with people just crazy enough to notice him?