Certification happens once a year. Every nation certified democratic in the prior year votes in the certification process for the next year. A "yes" vote certifies that a nation is governed democratically, a "no" vote withholds certification. There is no abstention: every eligible member votes on every nation but their own.
Certification is not permanent. A nation that loses its members' confidence loses its vote the following year, the same way it earned it: by the judgment of its peers. Ultimately, the goal is for all nations to be democracies, thereby advancing the cause of world peace.