"A Fenderson is a member of a family you can join by saying you are one. Yes, anybody who wants to be a Fenderson can be a Fenderson. Just say these three words, 'I'm a Fenderson.' It's as simple as that."
-- Fenderson Discordian Graham Trievel, in the Introduction of Principia Discordia.
You can also be made a Fenderson because someone says you are one...
I, Stompy Fenderson, hereby declare that you are a Fenderson.
So... now that you are a Fenderson, just add 'Fenderson' to your existing name, or give yourself a new name and add Fenderson to it.
The rule of being a Fenderson is this: "you must at all times keep in touch with other Fendersons". This is best accomplished by making everyone you meet a Fenderson.