How do you create a ribbon emmitor?

How do create a ribbon emmitor like those used by the Avatar of Vengeance and the Rejuvenation effect?

Ribbon emitters and particle emitters are essentially the same in how they are placed and set up, so reading a tutorial on particle emitters will tell you most of what you need to know, or at least give you some background informations. I'd suggest doing one of those first, since they are better documented. Once you are familiar with particle emitters, it's pretty much downhill. Just copy the template from a ribbon emitter (which you can copy out of rejuvination or something) and change the values how you want. If you get stuck on something more specific, I or someone else can help you with it.