As far as I can recall the "original" process, it involved at least one step of deliberate self-poisoning, fairly far along the lich-track.
Basically, the caster would create one of the most vile magical poisons ever concocted, specifically tuned to his own body and spiritual essence and then, during a particular ceremony, he'd drink it straight off.
He then gets a saving throw vs. the poison. If he fails, he dies (and it might also be a death effect; I can't recall specifically). If he succeeds, he becomes a lichnee - mostly dead, but slightly alive. He basically stops aging at this point except mentally (similar to the druid or monk ability, actually).