How to turn on voice dictation in Yosemite using a voice command?
Apple says:

Enable Dictation Commands

Before you can tell your Mac what you want it to do, you need to turn on Dictation Commands.


  When you enable advanced commands, you can also create your own
  commands by clicking the Add Command (+) button. This lets you link a
  spoken phrase to an app, a menu item, a keyboard shortcut, or an
  Automator workflow.