You must be a member of ZU for over a month and have at least 50 posts, and then you can apply to take a test from some of the clan leaders or summat.
Depending on how you perform on the test, determines what clan is best suited to you, Imean, I thought I would be put in the Nayru clan, but was put into the mighty Farore clan, and the Farore clan rules lol!!
There is more infor on this in the Clan Faq of the Clan HQ.

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:: All around the country from coast to coast :::: People always say what do you like most? :::: I don't wanna brag and I don't wanna boast, :::: I always tell 'em I like the toast!::