This guide will help you get started with RetopoFlow 4.0.0α.
To start retopologizing in Blender after installing Retopoflow:
Either click “Retopology at Cursor” or “Retopology at Active”
All of the Retopoflow tools require some other object in the scene that the new geometry can snap to. You can also create a new retopology object yourself or select an existing retopology object, enter Edit Mode on it, and switch to any of the RetopoFlow tools in the toolbar.
Enjoy!
For a more in-depth explanation of how Retopoflow works, read the Retopoflow Mode page.