I think the easiest way to cover this would be to explain the process and each skill's part in it.
First you figure out the complexity of the spell and then try to match that with lore + other bonuses (I'm glossing over that bit, it's a bit more complex). Once that's done you (eventually) have to summon enough shifts of power to match the complexity. This is done as a series of simple actions similar to individual evocations where the caster draws as much power as he likes (any amount between 1 and his conviction plus any power bonuses for no stress, each one after that costing one additional stress) and then attempts to channel that into the spell rolling his discipline plus any control bonuses and attempting to get over the amount of power drawn. The drawing and controlling of power is done repeatedly till the complexity is matched and the spell works, or until the caster mucks it up (which is usually pretty bad, but depends on how high the complexity is and how far into the spell you are).
Does this answer your question?