This wonderful CodeProject work presents a very advanced HTML renderer based on
A Professional HTML Renderer You Will Use
Even though I would advice to use this component as is and forget about such a rudimentary format as RTF, the code you want is contained in this project, apparently.
Now, as I thinks it's pretty likely that your ultimate goal can use the renderer component, I would recommend to start all your questions from explanations of your ultimate goals. This way it's way more likely that you can get much better advice.