It will be a lengthy procedure. You must read the code and refactor any areas that do not adhere to the SOLID principles into new classes. The refactoring process can be aided by using a Visual Studio add-in like Resharper (
http://www.jetbrains.com).
In an ideal world, you'll have a large number of automated unit tests to ensure that your changes don't break the code.
Additional Information
You must discover methods that are related to one another in the primary API class and develop a class that more explicitly defines the operations that the method does.
Ex.
Assume I have an Address class with different variables for street number, name, and so on. This class is in charge of inserting, updating, and deleting data. I could create a method named GetFormattedPostalAddress() that returned the formatted address if I also required to format an address in a specific way for a postal address.
Alternatively, I might rework this method into a class named AddressFormatter, which takes an Address as a function Object() { [native code] } parameter and returns the formatted address via a Get property called PostalAddress.
The goal is to divide various responsibilities into distinct classes.