Melt chocolate in a microwave-safe bowl in 30-second intervals on High (100%), stirring between heating. Do not overheat. The bowl should be almost cool to the touch. Overheating will quickly cause the chocolate to burn. Heat corn syrup in a separate microwave-safe bowl on High (100%) for 45 seconds. 

Pour syrup into melted chocolate and stir with a rubber spatula until completely blended, about 30 generous folds of the spatula. Make sure the spatula and bowl are scraped clean of white chocolate. If any white chocolate has not made contact with corn syrup, you will have lumps in your finished modeling chocolate. Watch for white chocolate streaks while you stir. Mix until the streaks are gone. Do not over-mix. If you over-mix, you will notice your chocolate becoming crumbly and falling apart.

The next day, cut the block of modeling chocolate into knead-able sections. The block will be greasy. It needs to be kneaded. Dust a little cornstarch on the table to knead if chocolate is sticky. If the block is too hard for you to knead, heat in microwave 5 seconds at a time until you can begin to easily press your fingers into the block with less force. You will notice the block changing into a silky and pliable clay-like ball. It will be a light ivory color after it is kneaded. This is now Modeling Chocolate. Cover with plastic wrap (Press-n-Seal is best) and drop in a resealable plastic bag for storage. Store at room temperature for 3-4 months before the oils begin to dry out, or in the freezer for up to 2 years. 

To color: Knead in gel paste or liquid paste color of any kind. The water-based icing colors will be perfect for the job. Start with a small amount, and add a little bit at a time until you get the color you want.