A lich is a miserable creature.
I'm not playing the tired refrain that "Living forever is a curse and a burden." It's not. Life is usually desirable. Life unending is a desirable thing.
But lichdom is not. Lichdom is not eternal life; it is eternal death.
Lichdom is only possible for the truly mad and the truly obsessed. One does not, cannot, become a lich if you have good intentions sadly bounded by brief mortal constraints. A lich has an obsession that they cannot give up, not even in death. Some love; some vengeance; some task; that must persist, long after their muscles give out, their skin is torn away, and their bones crumble into dirt.
Many have attempted to attain a state of lichdom. Few have succeeded. Those who have enter into a unique infamy. Their names are forever remembered by the Men whose ranks they have left, but also forever cursed. They are the devils of the stories the Men tell each other. Their names become synonymous with evil.
Becoming a lich is, well, not secret. The Tower Gnostic probably has a text that explains the procedure. It's just difficult, lengthy, and unlikely.
The process goes thusly...
Step the First
Before death, a lich's diet must be one of wood*: nuts, pine needles, seeds, roots, and bark tea. This diet must be strictly maintained for somewhere between 3,000 and 3,600 days. This diet reduces body fat to nothingness. The would-be lich is literally mummifying himself.
Over this time, the intake of liquids is curtailed and, finally, eliminated. A lich must die of thirst.
* The diet varies from tradition to tradition. Sorcerers practicing weald magic have a different diet from those practicing weird magic, for instance.
Step the Second
Only master sorcerers are afforded the option of lichdom, for reasons which will soon become clear. The would-be lich must memorize and maintain a rigid proscribed panoply of spells for a period of at least forty-one days. (In D&D terms, this is something like "a wizard of at least 15th level must fill all available slots with spells, with at least two duplicates at each level.")
Holding spells and not releasing them is incredibly difficult. The process requires strict concentration and meditation, lest some cantrip slip out unbidden.
This burden literally causes changes in the sorcerer's physiology. Their brains cannot handle the weight of the spells, and twist and grow cancerously. They bulge painfully against the skulls of the sorcerer.
During the period of spell memorization, when the brain-swelling becomes too intolerable for life, trepanning is required. This step requires the help of aides. The flesh of the center of the forehead is carefully cut away and pried open, like a flower or a citrus fruit. Then, a boring utensil makes a hole in the skull. A runic inscription is then written around the edge of the exposed skull. If done correctly, this inscription magically calcifies the lich's skull--it is no longer vulnerable to any physical damage.
Though this is (obviously) incredibly painful, no pain-dulling drugs are allowed the sorcerer. Total calm must be maintained, lest the spells escape and ruin the process.
Step the Fourth
In the last week of spell memorization, the sorcerer must cut all water from their diet. As the sorcerer is dying of dehydration, a deep state of meditation must be maintained. It takes about a week to die once all water is refused, and the meditative state cannot be broken during this time.
During this time, the phylactery is made.
The phylactery can be anything, as long as it's an object of complete obsession for the lich. It is usually a love letter, or a shell, or a broken dagger. It's almost always transitory and vulnerable--that's why he's obsessed with it. It is some object of significance. It is the reason that the lich has been undergoing all this suffering. A lich does not choose to be obsessed with the item. The item is obsessive. It drives the lich.
During the last week of dehydration, all the sorcerer's attention is placed on this item. It is held and cradled and loved and obsessed over. Nothing else is touched or handled during this crucial period.
Step the Fifth
Now comes the dying. During this time, the need for complete concentration is greatest. Normally, all spells that a sorcerer has memorized come pouring out of their brain upon death. The lich must not, cannot, let this happen. If he does, all is ruined.
A save (Con? Fortitude?) is necessary for each of the spells memorized as the would-be lich dies. If even one save is failed and a spell is released, the entire process is ruined.
Step the Sixth
At this time, the sorcerer has either interred himself or is relying on aides to inter him. The place the corpse is buried is important. It must be soft and rich earth, thick with worms. Good crop ground.
The would-be lich's corpse must be devoured by worms and other detritivores. This step is only compete when they get to his spell-swollen brain.
Step the Seventh
If the phylactery was truly obsessed over, spells in the dead brain get confused about where they belong. Should they be in the rotting material they're currently trapped in, or should they be in the phylactery? Where is their master? What are they?
This epistemological concern is the catalyst for the seventh step.
As the worms eat the brain, the spells get confused and begin to effect a change on the detritivores. The spells incite change and mutation. Over the period of many years, the worms--their lives unnaturally preserved--begin to merge and shape themselves together. They try and cross the gap between "worm" and "the sorcerer."
With difficulty and pain, these worms form themselves from many into one, and from death into undeath. Using the phylactery as an emotional anchor, the lich is given a body.
And there you have it. Now you know the deep secret.
Insanity and evil begets eternal life. Virtue has no such counterpart.