The Basics
First off, there is in-game documentation. You can craft yourself the book Sanguine Scientiem, a pretty decent manual only slightly hampered by being written "in character" by a fictitious mage.
Second, briefly, getting started with demonic will:
Growing crystals
Crystals can be grown in a Demon Crystallizer. In order for a Demon Will Crystal Cluster to grow, the chunk in which the Crystallizer sits must have at least 90 points of some flavor of Will Aura: Raw, or one of the four specialized ones. There can be a maximum of 100 points of Aura in any given chunk.
For lack of a way of creating the others (other than rarely by chance), you'll be starting off with Raw Will Aura. To put Will Aura into your chunk's atmosphere, you need to burn Will crystals or Tartaric Gems in a Demon Crucible. You can use the Hellfire Forge to transfer Will from Demonic Will items into Tartaric Gems. Tartaric Gems will usually tell you how many points of Will they hold, so you can estimate your way to those 90 points.
Even with chunk aura saturation at 90-100, your Crystal Cluster will take some random amount of time to appear. I didn't use to know about the 90 point minimum requirement, so I often spent a long time waiting for a crystal that wouldn't happen.
Your chunk has 5 different aura values: one for Raw, and one each for the 4 others. It can and will have different, independent values for each of those. Crystal growth doesn't depend on the total being 90, it insists on a single aura value being a minimum of 90.
Once grown, you can smash a Crystal Cluster with a Diamond-level Pickaxe. For a freshly grown Cluster, this will net you one Crystal. But don't do that yet!
A Crystal Cluster consists of 1 to 9 crystals. The first one "costs" 90 (or maybe 100, depending on version) points, the other up-to 8 others are cheaper, usually 50. The more crystals in the cluster, the more you harvest. So whatever you plan to do with the crystals, you're usually better off waiting for the cluster to fill out.
Also, for the same reason, you're better off not smashing the cluster with a Pickaxe. Instead, if your inventory is holding at least 1000 points' worth of Will (in Tartaric Gems), then you can harvest a cluster by right-clicking with an empty hand. You will get the up-to 8 "secondary" crystals but leave the "senior" (straight upright) crystal intact. So you can keep harvesting crystals from a single cluster so long as you keep Crucible-infusing Will into your chunk.
As soon as you can afford to craft one, you should make yourself a Demon Will Aura Gauge. This will show you how much of each kind of Will is in your chunk's atmosphere. A little panel with 5 bars is displayed (by default) in the top left corner of your screen. I didn't see it at first because my The One Probe display sat on top of it. Having the aura values visible takes a lot of the guesswork out of crystal growing.
To spawn a new Crystal Cluster in a Demon Crystallizer, you need to get the atmospheric aura to 90-100 points. Depending on how odd a number of points you have in the atmosphere, it may not be possible to reach this range by burning another crystal. If burning a crystal would overshoot 100, the crystal instead just sits in the Crucible, doing nothing.
What works in this situation is to burn a Tartaric Gem. Will will be extracted until the atmosphere hits 100 points or the Gem is emptied or you take it out of the Crucible, whichever happens first. Thus, topping off the aura value from a Gem is a surefire way to hit the sweet spot for new cluster spawning.
An easy way to charge a Tartaric Gem is to place it into a Hellfire Furnace (in the rightmost, "fuel" position as usual) and burn crystals in a Crucible in the same chunk. The Gem will soak up all available aura (of its own type) from the atmosphere, or until it's full, or until you take it out of the Forge.
Once you have a comfortable handful of crystals, you don't need a Demon Crystallizer to grow secondary ones. You can use the Hellfire Forge and a bunch of Will to craft 4 crystals into a Crystal Cluster, which you can place on the ground or anywhere else as a normal block, where it will grow secondary crystals if atmospheric aura and time permit.
The Crystal related Rituals
Depending on your Blood Magic version, you'll have access to these two rituals:
Placement: The folks I've seen on YouTube usually set up the GotFS ritual and then later add the CotFC a few blocks above, or maybe even below the first one. It's very obvious from the layout of the Ritual Stones that the author intended these two rituals to be placed smack dab on top of one another - the stones fit together like lock and key. Or at least, he made it obvious and possible. Realistically, this means placing the CotFC Master Ritual Stone directly on top of the GotFS one. Because you end up with blocks all around, that makes the first Master a bit hard to access; you'd need to approach it from below. What I did was to fire up the first ritual, constructed the second one and then started that. I had no intention of ever stopping or controlling the Rituals, so that worked out OK for me.
Getting the different "flavors"
With the above out of the way, it's still not obvious how to arrive at crystals for Corrosive, Destructive, Vengeful, and Steadfast. The following fact will help:
When a chunk's atmosphere has 90-100 points of any kind of Will, a new crystal cluster can grow in a Demon Crystallizer. There's a 90% chance that new cluster will be of the same kind (Raw or one of the special 4) as the aura that's thick in the atmosphere, but also a 2.5% chance for each of the remaining 4 types to spawn.
So to get a different color, you need to keep trying to spawn new clusters, hoping for that 1 in 40 chance of getting the color you want. This is tedious, but having the Gauge to tell you what the aura level is helps you be aware of what's going on, and being able to top off the aura value from a Tartaric Gem makes it a controllable process.
I recommend multiplying Raw crystals by the method described above until you have a comfortable stash from which to charge your Gems. Continually starting new clusters and breaking them is a process wasteful of Will, so you want to be prepared and have some to spare.
UPDATE:
According to the mod author's change log, as of version 2.2.9 the aspects work differently.
My thanks to /u/MuteTiefling for pointing this out and /u/Ligands for digging up the details!
there's a new ritual for forcing a different aspect: "Resonance of the Faceted Crystal." No link, sorry, it's so new there isn't a page for it yet. And, unsurprisingly, I can't tell you how it works.
The Crystallizer no longer has a chance to give you a random-aspected Crystal Cluster; and
Crack of the Fractured Crystal will harvest secondary crystals as soon as they appear. Well actually, it will snap them off and drop them. I use a Ranged Collector to pick them up. This ritual is practically a must if you want to automate crystal production.
This ritual makes "infinite Will generation" economically possible. It costs less to spawn a secondary crystal than that crystal releases into the atmosphere when burned, so by harvesting secondary crystals and feeding them into a Crucible to grow new ones, you can keep growing your pile of crystals. I think it's possible without the ritual too, but there's a smaller "profit margin." So this ritual is highly recommended if you're planning to produce a lot of crystals.
Gathering of the Forsaken Souls will kill mobs within its 27 block radius range, similarly to the Well of Suffering ritual. A killed mob doesn't contribute Will, so don't expect a running ritual to pop up crystals all over the place. A killed mob just speeds up (instant?) the growth of secondary crystals on a cluster, provided there is enough Aura available in the chunk. Apparently, this ritual also decreases the point cost for the growth of a secondary crystal.
You can go chasing mobs in the night with this rinky-dink sword, but I recommend building a (manual) mob grinder where the mobs get lined up for you to safely hack at their ankles.
Your Sentient Sword can be enchanted to make it less crappy. I highly recommend Mending, as without it your sword will break after every few hundred Will. That way you won't waste Will on re-crafting the weapon needed to gather Will.
Your crappy little Sentient Sword gains attack power (from 6 to a max of 9) if you carry enough Will around with you.
Hints:
Use your Hellfire Forge to cook those Wills into Tartaric Gems, which are basically portable capacitors for Will. You need the Gems to transport Will into the Forge to craft later items.
Craft a Hellfire Forge and then a Sentient Sword. You can then gather Will by killing mobs with this weapon.
Craft a Rudimentary Snare. You need them to put monsters into a state where killing them will reap a tiny bit of Demonic Will. I tend to fumble this, so I usually craft 8.
https://www.reddit.com/r/feedthebeast/comments/8mjz74/secrets_of_blood_magic_demon_will_crystals/