Editor scripts into external assemblies? how?

I have a class that I'd like to have a custom editor for. I have the class working in an assembly DLL ok. Now, how would I go about doing the custom editor? Can I put it into the same assembly? As I get it, it needs to go into (an) Editor folder, so do I have to have two assemblies, one for the main class, one for the editor? Or maybe there's some structure to the assembly itself? So confused!