How to create a native OS X plugin from a static C++ .a library

I have a native OS X static C++ .a library that I want use in a Unity project for OS X.

I have the source and know that I need to export "C" bindings of the methods I want to access.

What's stumping me is how to get from the .a file into a bundle. The Unity docs explain that you have to use Xcode to create a bundle. There's where I'm stuck. How exactly do you do that?

Specifically, after creating an empty bundle in Xcode, what do you do to bring the .a into a bundle?