The info is near the bottom of this article http://www.issfanclub.com/node/8047
It basically says to send in your report to one of the ARISS QSL bureaus with "25th Anniversary Certificate" on the outside of the envelope and wait.
Kenneth - N5VHO
Support ARISS http://www.amsat-na.com/store/donation.php (select "Human Spaceflight (ARISS))
how do i pay for postal cost?
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Include a couple of IRCs (International Reply Coupons) or "green stamps" (dollars) with the self addressed envelope.