I have found after making modeling chocolate for almost 20 years, that the most important thing in being successful at making perfect modeling chocolate every time is making sure it cools to between 91 and 92 degrees Fahrenheit (33° C)  before adding the corn syrup. If you add the corn syrup to chocolate that is too hot you'll end up with greasy modeling chocolate. See my troubleshooting on how to fix your greasy modeling chocolate. To measure the temperature, you will need a good thermometer. I use Thermapen thermometers because they are extremely accurate and register the temperature much more quickly than any others that I've tried. Thermapens are not cheap, but after throwing away so many less expensive thermometers because they just didn't work, I was so happy to find one that I love.  I've owned one for about 15 years and just got another one from Thermapen. If you add the corn syrup to chocolate that is too hot you'll end up with greasy modeling chocolate. See my troubleshooting on how to fix your greasy modeling chocolate.