After you have the membership, use the methods I discussed a few days ago to search for a tournament. Usually the tournament announcement will tell you how to sign up, and how to pay the entry fee. Sometimes you have to mail-in the signup form and check, but nowadays almost everyone moves to online. You can fill up form and pay online directly. There are some tournaments you don't need pre-signup. Just show up and pay the entry fee onsite.
As some old saying says, do it once or twice, you will know everything.