I am currently judging online about once a month, however it is for the Blackmoor DnD campaign and not the Fudge Deryni system.
Online play conceptually is the same as table top gaming, except that the participants connect via the internet. One person acts as the Moderator/Game Master (GM)/Dungeon Master (DM) and the rest of the group participate as the characters involved. Sometimes the GM creates his/her own scenarios and sometimes the GM is merely using someone else's scenarios. Sometimes the players just uses pre-generated characters and sometimes the players bring characters they created using some sort of character creation rules. Character creation rules are almost always specific to the campaign setting, here that would be within the Deryni universe.
Grey Ghost Games (My apologies if I didn't get the name entirely correct, Anne) published a Deryni Adventure setting which is geared for the FUDGE ruleset and can be adapted into other rulesets. I have only played in this setting twice, both times with Anne Dupois and Katherine Kurtz at WorldCon in Boston.
Other than enthusiatic players, the main keys to getting a good game going are 1) good scenarios and 2) a good GM. You also need time to play, which is getting harder and harder to find these days.
Let me know if you have any other questions.