I'll sketch this VERY lightly.
The idea is that thaumaturgy breaks down into preparation, and into powering.
If you're casting a small enough spell, you can skip the prep phase: it's assumed you're able to put that all together real fast. (Harry's done this with quick-whip-'em-together tracking spells before, among other examples.) I think I recall that Lore helps determine what your "no prep threshold" is, in that regard. If your spell's complexity is over that threshold, you have to spend some time (possibly a lot of time if you're doing something gigantic) essentially telling the story of the spell, as you gather together the parts you need or wait until the Stars Are Right or whatever.
Once you've completed preparation, your spell is "constructed", but unpowered, like a toaster that isn't plugged in yet. So you get to work powering up the spell. Powering up the spell works like Evocation -- almost exactly, with a couple small but important differences that are best left for the book to cover -- but you're able to "chain" the evocations together, done back to back, as you funnel power into the spell (either slowly or quickly depending on how much power you can grab onto and control from round to round). Eventually, you fill the spell all the way up, and it's cast, unless something goes really badly during your efforts. (If you're not under any threat of violence or other time pressure while casting, powering up can all be skipped; you built it, so you can cast it at a nice leisurely pace, no point in rolling.)
That's about the size of it. Lore is super important for spell preparation, as you might expect. It's an interesting skill, with a number of small but potent utility functions that are spread all throughout the spellcasting system.
