This is a practical guide for the tricks you can use to do big rituals. It assumes you are a wizard focused in ritual magic, though other ways are certainly available;
First of all, the basics
Make sure you have a Lore of +5, a conviction of +4, a discipline of +5, specialization in Spirit Power and in the type of thaumaturgy you like best. You should also have a +1 power/control for spirit offense, if you're into offensive ritual magic or spirit defense if you're in defensive ritual magic and a +2 complexity focus for the ritual magic of your choice.
Secondly, time
Even if your GM is relatively lenient, you won't want to spend too much time on a ritual; it's impractical and you'll be missing a lot of the action. Ritual magic that needs multiple scenes to prepare and cast can level entire cities; you should not use it without DM approval and without being absolutely certain of what you are doing. For now, we'll look into single-scene rituals.
Third, mundane preparation
While you can use the half-assed approach of doing a quick and dirty ritual with what you have at hand and without any preparation, this limits you to your base complexity. Even mundane preparation can have a huge impact on rituals: you are allowed to use aspects in a ritual but while you usually have to invoke them, skill declarations can create them and thus let you tag them for free. Using your skills, take the exchanges needed to make declarations/applications of aspects via maneuers. To apply a sticky aspect, you need 4 shifts which should be easy for your level of skill. Generally speaking, amassing preparation via sticky aspects can be powerful but don't overdo it; taking too long may prompt the DM to change the scene with an inconvenient phonecall of a friend asking for help, in which case all your sticky aspect preparation goes away. (This does happen to Harry Dresden. More than once.) So better stick to no more than a few easily justifiable aspects like the following:
"Gathered my Full Power" or "Believe Absolutely In What I Am Doing" (Conviction)
"Mind Clear Of Distraction" or "State of Meditation" (Discipline)
"Perfectly Drawn Magic Circle", "Thematically Appropriate Ritual Components" (Lore)
"Costly Ritual Components" (Resources)
"Aide from Wiseass Advisor" (Contacts or Intimidate or Rapport, depending on the occasion)
Harry Dresden has used all of the above in one time or another. For especially big rituals, like his summoning and binding of the Erlking, he used many of them at once.
Fourth, Calling Power
Generally speaking, heavy-duty rituals need lots of power and most people skip to the next step and do sacrifice to find it. However, it is possible to tap far more dangerous forces for extra personal power without sacrifice.
The simplest way to do that is directly tag an existing aspect or time with ritual significance such as "violent storm" or "stroke of midnight" or draw power from sources of magic such as Sponsor Debt or a Leyline. This gives +2 per aspect thus tapped and per point of debt drawn from sponsors.
You can also call your own power; Evocation can apply aspects just like mundane preparation. But unlike mundane preparation, spells to call more power both cost stress and have the danger of getting out of hand and blowing up in your face if you draw too much-plus you need to make the Evocation apply thematically appropriate aspects. For our hypothetical wizard, here are a few Rotes that are thematically appropriate for assigning aspects to yourself to use in rituals:
Leyline Tap: 6 shift spirit evocation, requires place of power; by drawing on a leyline or other place of power, you apply two sticky aspects to yourself. Effectively, you link with the leyline using your magic in order to draw more power for your spells.
In the Name of [insert supernatural heavyweight] 6 shift spirit evocation, requires a Name; by uttering the Name of a powerful entity such as an Old God, a Faerie Queen or an Outsider, you draw power from it and apply 2 aspects to yourself. Effectively, you link with the entity using your magic in order to draw more power for your spells. Unfortunately, aspects applied this way are always colored by the type of entity you invoked and usually draw its attention.
Tap Natural Disaster: 6 shift spirit evocation, requires a natural disaster; by drawing on the immense energy of a natural disaster such as a storm, earthquake or volcano, you apply two sticky aspects to yourself. Effectively, you link with the natural disaster using your magic in order to draw more power for your spells. Unfortunately, using natural disasters is not easy both because they don't come very often and because aspects applied in this way are always colored by the type of energy you used and may have destructive consequences.
Hour of Power: 6 shift spirit evocation, requires metaphysically important moment; by employing a minor invocation of the metaphysical significance of a certain time such as midnight, dawn, dusk, new year's eve, all hallows' eve or similar, you apply two aspects to yourself, drawing more power for your spells.
Effectively, with the above spells you are using an aspect you could normally tag or invoke for a +2 as a justification to use aspect-applying maneuers to yourself to create (and then tag) multiple aspects. Technically, you don't need such justification but usually a DM will require applied aspects to be sticky before you tag for a ritual. You'd need an 8-shift evocation to apply two of them while using the existing aspect as a stepping stone, you need only 6 shifts. In both cases, even an average wizard can employ four such spells in a scene to get 8 aspects of invoked power at the cost of expending his mental stress.
Calling power is exhausting and thus very dangerous; doing so when you aren't sure you are save may prove unpleasant. Still, LOTS of people do it, from Harry himself to horribly evil people with torture spells that not only create aspects for use in evil stuff but also force sacrifice on other people (see below). Perhaps the biggest such attempt was Cowl's blackout that applied a "fear is power" aspect on hundreds of people all over Chicago. That was a serious spell on its own right but it enabled Cowl to tag "fear is power" hundreds of times, getting hundreds of shifts towards achieving the Darkhallow. (it wasn't enough on its own but it helped)
Fifth, sacrifice:
The fastest, though not the easiest or the most moral way to add to a ritual is sacrifice. The simplest sacrifice is expending Fate Points to, in terms of mechanics, invoke aspects, either yours or of the scene or even on others, for a +2 per Fate Point spent. The second simplest sacrifice comes once you remember that Wizards will always heal completely from anything save Extreme Consequences... eventually. So you can deal mild, moderate and severe consequences on yourself to gain power. Generally speaking, it is safer to only inflict your mild consequence which will heal by the next scene. Some wizards even have two or three mild consequences (depending on skills and stunts) to invoke.
Third, you can spread the sacrifice around to more than one person. This is finding helpers, people that help you in the ritual. Just the people of your own group could sacrifice a Fate Point and a mild consequence each easily to support your big spell. In a group of four, this light sacrifice can give you +16 shifts. In a group of hundreds such as the White Council, it can level cities.
Of course, you can do heavier sacrifice. Sacrificing your life is at least +20 shifts. So is sacrificing the life of anybody else. And you can do more than one blood sacrifice; you are only limited by your willingness to kill others to gain power.
The Darkhallow is one of the biggest attempted sacrifices in the books; by destroying hundreds of powerful spirits (i.e. spirits that do have consequences to be dealt to them) and eating their energy, you add to the power of your ritual. Of course, you have to call and bind those spirits first - the Kemmlerites took advantage of all those spirits being hunter spirits that would follow the Huntsman and thus only had to affect the Erlking to affect all of them but most times you have to do the calling and binding the hard way.
Sixth: adding up the strength of the ritual
Time to see how powerful the ritual you are about to cast actually is - represented by its total complexity. So, you have a base effective Lore of 8, you did five mundane declarations, you cast four callings of power resulting in eight more aspects and you spent an exchange along with your friends to do a mild sacrifice each, invoking four total aspects and inflicting four mild consequences.
That's a grand total of 8+10+16+16 = 50 total shifts. This is about the maximum ritual strength you can realistically gather in a single scene. It's enough for average rituals such as instant-death on everyone in a medium building from across the continent, gating a group of people from Chicago to Endimburg, applying a powerful inherited curse that lasts for generations or tossing the Empire State building a couple of city blocks.
If that is not enough for what you want to do, you have to go back and do the research for a long time, add +1 complexity per scene you thus spend researching, and THEN do the preparation again; because sticky aspects normally only last for a scene, you can't add up more of them across multiple scenes unless your skills are significantly higher and you can make them last longer.
Sacrifice on the other hand DOES add up. You can sacrifice someone now and a hundred days from now and if it is for the same ritual, they stack. Ditto for smaller sacrifices.
Last part of the preparation is to have a symbolic link to the target. For any creature, a part of itself (hair, nails, flesh, body fluids) will do. That's all but impossible to get for most Nevernever creatures except fae because their pieces melt to goo when dead - therefore you go to option 2, their True Name. Third option is to mark the creature; you can slip them a token ahead of time, an object you can use for the magic to track, or a piece of an object they still hold. In that case, you need to use up 2 shifts of your spell for area effect because the magic will track the mark but if it is single-target, it will only hit the mark itself, not the creature. Hence AoE to affect all in the same zone. That has the unfortunate drawback of collateral damage, if you are doing offensive spells. Whoops!
Seventh and final step: Casting
Once you are prepared, usually in the same scene for the short ritual we've been preparing for ten exchanges, it is time to cast. Once you start casting, you begin to actually summon the power you prepared for; the preparation so far was for your spell construct and your own mind and body to be able to hold the power. You still have to call it up.
So you begin calling shifts of power up to your conviction (+4) each exchange and control them with your discipline (+5). Unfortunately, you can fail the control roll and take backlash or fallout-which is bad. So you call less than your full conviction, say, 2 shifts per exchange. This gives you a fairly good chance to call all 50 shifts, over 25 exchanges, and cast the spell.
Make certain someone doesn't bump your elbow (or the metaphysical equivalent of it) while you are casting. If you lose control of the power you have gathered, it is unleashed as backlash and fallout. And while the 5-10 shifts of evocation may, at most, blast a small house, the 50 shifts of a medium ritual will annihilate anyone in a couple of city blocks.
