Battery 24v to 12v diagram?

[mocman]

I have replaced my evinrude 12/24 volt with a new motor guide 12v that came off my boat I sold. I have a 3 wire plug. I need to know how to hook this up from parallel to series. I also would need to know what to do about the plug? I know how to hook the batteries up in series but what do I do with the other wires on battery end?