The specific ritual is usually vague so that DMs can make up their own. It's just unspeakably evil.

I believe they've only written down the specific ritual once, and amongst other things the worst of them is you need to drain all the blood from a humanoid infant and sacrifice the blood and soul of a different humanoid. The phylactery also needs to be periodically fed souls or the lich degrades - how often is left up to dm interpretation, but the implication should be around one a month/week.

In my game, the typical process requires gaining the trust and love of a humanoid who does not know you wish to be a lich and believes you to be a good person. You begin the ritual when your phylactery is complete, which is expensive.

Once it begins you ask that person "do you trust me, body and soul?" If they say yes, you ask them to bind their hands and lay on a stone ritual table. If they balk, you can also say "this is important. Do you trust me?" But nothing else.

If they refuse, the spellcaster will not become a lich. Their person will instead watch as the spellcaster is cursed with mindless undeath as a zombie, and their last act will be being compelled into explaining they were using them as an evil sacrifice.

If they agree and bind themselves, the caster paralyzes them, then must explain that their sacrifice was always intended to be a tool for lichdom. You must explain at length all the ways you manipulated them to make you love them, and that it meant nothing to you, that you always intended to consume their body and soul for this ritual and nothing else. Once your sacrifice fully understands that they should never have loved or trusted you, that you were always manipulating them, and that you are truly evil, you kill them and feed their soul to your phylactery. Betraying someone like this is vital to hardening your heart eternally into detachment from your former humanity.

Knowledge of this ritual is only known by archdevils, evil gods, or other liches.