This tutorial won't cover the modeling techniques to create the high resolution model, so create the high polygon head and export it.
This image shows the beginning of creating wrinkles in the face.
Notice that small details such as rough metal surfaces, even though being a "normal map task" will be created in photoshop with Nvidia plugin not with Zbrush. You will better understand it later.
When subdiving during the high poly creation, I recommend disabling the "subdivide smooth modifier" button in the first subdivisions of the mesh, that way it won't smooth all the mesh making it smaller or very rounded, but that will generate corners in the mesh you'll have to smooth. It will be also useful to put the texture off and see the mesh in a single color better deal with the details.
Once the high resolution model is done, export it within the highest subdivision level. Maybe you need to disable the "Group" button in the Zbrush exporting options, so you won't have separate objects.
Normal Map Creation
Import the model from Zbrush, apply a smooth modifier (you can activate auto-smooth and increase the threshold slightly as you look at the results) if the mesh seems too sharp or faceted in some parts and place the high resolution model in the same position as the low-poly model so they are overlapping.
As you can see in the image, these are my two meshes after overlapping them. Notice the high resolution is only the not mirrored parts, re time with the texture later, that's why I have recommended to create the high-poly using the whole mesh!!!!!
Now select the low-poly mesh and Rendering->Render to texture (shortcut:0) then using the parameters of the image below hit Pick button and select the high resolution model.
In projection modifier parameters you must update the cage and increase the amount till no red parts are shown in the normal map when you hit render.
After some test you can increase the resolution of the rendered map to the desired size to get the final normal map texture.
But normal map creation has not finished here if you want great details. You'll have to touch up the normal map to improve it and to hide any possible overlapping.
You may also use your color textures created before, apply the Nvidia filter
) to them and use the layers as overlay with the desired opacity putting them over the base normal map created with max. This part can make your model have great details!
To understand it, we could say that big details (displacement) are made with Zbrush and small ones (bump) with the Nvidia filter.
Notice that some parts of the texture are very difficult to be done using a high model. For example, it would have been hard to high poly the metal cables at the sides of the model according to the texture it had. So I created the normal texture for it using a touched up version of the texture and applying the Nvidia filter. Remember that this filters works with black&white parts to create the heightmap 3d look. You can see the result here: