Capture a unowned wild level 5 or higher. You are not guaranteed to win a crest just because you get a wild. You can also purchase a chest for 90 gems.
According to Anna James, the chance of getting a crest is roughly 10% as long as the wilderness has not been attacked within the last hour. The chance decreases depending on how many troops are defending. ~Simba_Rawr (Galatine 3)