All you need to do is hit up their donation page, choose an amount, and send away. You will be put on the pre-order list for the album when it releases in a few days. All proceeds will be fired off to the Children's Cancer Association directly, meaning it's fully tax deductible. Top contributors will also receive an extra gift, from plushies to art commissions.
For more detailed information, hit up the Bronies for Good website here!
UPDATE: The actual donation page isn't updating people into the album list, if you donated directly without following the instructions on the bronies for good site, send an email to firstname.lastname@example.org with the name you used on GlobalGiving and the exact amount you donated.