Socialism has always had internationalist ambitions, but what those ambitions should be and how to rethink them in the 21st century remain open questions. Before his tragic passing in 2020, Michael Brooks talked about a new kind of cosmopolitan socialism that would be appropriate for our time. A How To Guide To Cosmopolitan Socialism builds upon Brooks' vision to argue that we need a left which knows no boundaries and recognizes the fundamental moral equality of all individuals on the planet while securing the material conditions for their flourishing. Only such a sweeping vision can successfully combat the forces of reaction and violence confronting us today.