calling function from native library.

Hello,

I am new to xamarin. it would be of great help if someone could guide me step by step regarding how to include my static library (.a) in my project and call the function from it . The function in the library is a simple c++ function which returns a string. I read this link http://docs.xamarin.com/guides/ios/advanced_topics/native_interop/ but still confused.

Thanks