Modeling a portrait is probably the most challenging thing to model, but it can be done, with lots of practice
Years ago (way back in Aspire 3) we created a set of tutorials dedicated to modeling George Washington's face - you can watch the set of videos here if you like: https://www.youtube.com/watch?v=DhlIuhR ... aMGxjhWUjb
Please bear in mind that this was filmed using Aspire 3
- so you will notice the component tree will be different to how it is today - BUT
the concepts & techniques behind building up all the parts and how to sculpt your model pretty much remain the same) This is intended for the more advanced/experienced relief modeler, so if you've not touched on any form of modeling then I highly recommend that you go through all of the standard 3D Modeling tutorials first before jumping into portraits.
Hope this helps,