Make sure the pan is completely dry. If not, it will cause the sugar / cornstarch to stick to the bottom and sides, making it really difficult to completely integrate everything else. It can also leave you with pieces of sugar later, which will end up ruining your taffy.
Okay, ready with the warning! Now take your sugar, salt and cornstarch and put them in (dry!) Pan. Stir them well until there is no difference between them. It doesn't take long before it happens.