Basically there are three parts I've been using. In summary I try to reuse models or topology I've already setup using lattices and looking at face parts in isolation.
First rough sketch and model using existing parts. Here setting up reference planes in Maya helpful to get look close to what we want, it being identical is not essential. Using a lattice on divided old parts of face to get a rough approximation then trying to stich pieces to roughly fit reference in Maya.
Then segment out the face into lip, eyes, nose, jaw/neck and work on their topology. Each part looked at separately using softmods, .. other quick blendshapes.
Last glue the segment pieces together, add a head. Start with making the front face gluing pieces that worked. Then add the back head using an existing back head. Then in Zbrush smooth out face close eyes and lips. Use Zbrush smooth and move topological brushes for closing eyes and lips. Having the reference in small window while visible in Zbrush was helpful.
Hope this was helpful.