Follow instructions "SKYMILES MEDALLION® STATUS MATCH CHALLENGE" page here http://www.delta.com/content/www/en_US/skymiles/medallion-program/skymiles-medallion-status-match-challenge.html and wait for an approval. It took about a month for me to receive an approval. In 90 days, you need to earn MQMs described in the instruction page. To qualify Platinum status, you need to earn 18,750 MQMs. Looks like you can be qualified for Gold if you can only reach 12,500 MQMs and 6,250 MQMs for Silver Medallion.