First, in order to Ascend you must be sponsored by an Elder God, that is, Rimbol, Smote, Mephiston or Siva. In order to be sponsored by them you must sacrifice a corpse in the presence of their high priest. This will allow you to join their religion.

Second, you must accumulate faith points with them, and then complete 3 "ascension quests" at every rank. You may get faith despite not reaching the next rank, but gain the next rank without completing the quests. You may only do the quests 3 times per rank.

The ranks for religion are as follows, with the amount of faith points required to reach them:

To get faith points you must sacrifice corpses or things of value. Objects or gold work, and 1k gold = 1 faith point. The higher level the corpse the more likely you will get a faith point upon sacrificing.

Once you reach the rank of Saint, you can ascend by typing "ascend" anywhere in the world. To ascend a second time, after you reach level 100, you do NOT have to do the quests or gain ranks again.

Upon completing the ascension quests, you will start over at level 1, remort 0, with a new subclass. You can read more about subclasses here.