Right....
First, your character must be a Mystic (preferably code A), with access to 124 magic points.
You can attain this througha large group casting, or by draining magic points from a dragon
you have bonded with...nothing too hard!!!
Then you must attempt to cast a permanent mystic Necromancy spell on yourself.
(incidently, if you fail, you'll die...). It should have a difficulty of 24 if your DM has
any sense of game balance...
If the spell is successful, with a permanent duration, you are now a permanent undead. Voile' - one Lich!!!
You will also be a mystic magic item, and people might want to drain magic points from you...
DM's should class Lich's as Artifact magic items...
If, however, the spell goes awry, you will be a lich for an hour at most, and then die.
It's up to you if you want to risk it...
P.S: Andre La Roche points out the spell would be an instant cast duration, since the spell creates a member of the undead, in the same way a flash of fire which burns stuff is instant duration.
All I can say is, I didn't read the rule book very much. : )
All the same, you may want to consider the implications of making it easy for your characters to become immortal (not to mention smelly).
Links to other sites on the Web