An working example of calling Native C code from a C# script in Android

Does anyone have an example walkthrough for writing a piece of C++ code with native functions, packaging this up in a .so and then calling it from a c# script in Unity? I have seen some information online, but they all seem to skip or go quickly over vital steps. I am having great difficulties getting native code running on android and any guides, hints or typical gotchas would be greatly appreciated. Thanks