It’s simple. You earn a credit for every month that you pay for your premium membership. To add a class to your Library, select the ‘Add to Library’ button at the top right corner of any class. This button will appear at the top of all classes when you have available credits in your account. To find out how many credits you have in your bank, you can click on the circular profile icon located on the top right of the page, the drop down menu will provide you with the amount that you've accumulated.
With the exception of multi part series classes (which can take 2-5+ credits to save in their entirety into your library) most classes will require 1 credit to save the class.
*** Multi part series classes will have more than one chapter/class with the "Add to Library" button ***