Not really advice, but ...
Basically all you have to do is call them up and join. Decide how much you want to donate and they'll be glad to take it. They're quite nice when you call them on the phone, and will take your pledge just about anyway you want to do it ... lump sum or monthly, credit card or check. Personally, I have them automatically charge my Visa each month.
Other than that, you don't do anything, nor do they other than send mail occasionally.