This is straight from the Wilton website, I usually buy chocolate melts at a local shop by the pound, they are fresher and tastier than wilton, so I use closer to 1/2 cup syrup to 1 lb chocolate.

Candy Clay

14 oz. Package of Candy Melts®
1/3 cup light corn syrup
Melt Candy Melts following package directions. Add corn syrup and stir to blend. Turn out mixture onto waxed paper and let set at room temperature to dry. Wrap well and store at room temperature until needed. Candy Clay handles best if hardened overnight.

To Tint: Candy Clay may be tinted using Wilton Candy or Icing Color. Knead in color until well blended.

To Use: Candy Clay will be very hard at the start; knead a small portion at a time until workable. If Candy Clay gets too soft, set aside at room temperature or refrigerate briefly. When rolling out Candy Clay, sprinkle work surface with cornstarch to prevent sticking; roll to approximately 1/8 in. thickness.

To Store: Prepared Candy Clay will last for several weeks at room temperature in a well-sealed container.