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Speak with the NPC Jessica Rogers to pick up this daily quest.  Speak with her again get your Ring Toss action bar.  You have ten tries per Darkmoon Game Token to complete this quest, but your attempts are cumulative so if you don't make all three in one token, you can pay another to finish it off.

This quest is the easiest of all the Darkmoon Faire games to complete on the first try.  Although you cannot completely control where the rings land and the turtle likes to move around randomly, he is generally pretty slow and the margin of error is wide enough that it is easy to land three per try.

With 6.1, the quest is not as easy.  The targeting reticule is much smaller, the cast time for Ring Toss is longer, and the turtle moves around more often and at a faster pace.  This is due to the addition of  Triumphant Turtle Tossing which rewards  Darkmoon Ring-Flinger.

The trick to completing the quest now is thus:  Dubenko always returns to the same position after he wanders off.  Position the targeting reticule at that location, then when the turtle turns around and begins to return to that location, that's when you use Ring Toss.  You can't land more than one because the turtle is a bit too quick for that, so have patience.  (Be the tortoise, not the hare! /lol )