I won't describe how to actually use the different tools in Photoshop, because that's very much artist related and out of the scope of this site.
A (tangent space) normal-map represents the local normal, or the orientation of that part of the face. If your brain can realize it and know exactly what combination of colours to use to draw it, you can draw it manually in Photoshop. My brain can't do that, and I'm sure the amount of artists that can do it is close to zero.
Since it's an orientation, it can be inferred from height information.
This is why you need a height-map first.
A typical height map is a grey-scale image, where white colour represents the highest height, and black represents the lowest height. All the greys in-between represents various other heights.
Using your texture as a guide, you can draw something like this with Photoshop (I used line, rectangle and brush tools):
After that, you can use a tool to easily convert the produced image from a height-map to a normal-map, I used this online tool to produce this normal-map:
I think you could also use NVIDIA Texture Tools for Adobe Photoshop to produce the normal-map.
- Use your texture as a guide
- Draw a height map
- Use a tool to convert the height map to a normal map
I know the aforementioned NVidia tool can produce a texture to a normal map. It's simple and fast, but the results are of a lesser quality than producing the height-map yourself.