Artistic license, Photoshop, and plenty of time.
I started off with an image of the 3D model so that the portrait would be the right dimensions.
Then using only the brush tool, different selection tools and the dodge and burn tool I made the facial features. I never used any filters in this project.
Next I added more grey. I turned down the saturation of the whole picture. Then used brush tool set on colorize. I panted around the eyes and the mouth with white and then faded it.
In my opinion the picture was too dull. So I gave him bigger darker eyebrows and used burn tool all over the face. At this stage I thought I was finished with the picture and stopped working on it for a long time.
The picture was still too dull so I used warp tool on each individual facial feature and gave him some expression plus a personality.
Next I used brush tool and gave him hair.
Because I like things to be very bold, thought the picture was still too dull. So I set the brush tool on colorize and painted the shadows bright blue. Then I faded it until it looked natural. I also painted other parts of the face red-orange witch I also faded. I looked at the picture with satisfaction and thought I was done.
I was wrong. Next I made the shadows extra blue.
Finally I used dodge tool and then darkened the background.
I thought I had done my best long before the project was over. There is always be room for improvement.
When making any portrait the eyes are the most important thing. Don't forget the highlights, don't forget the shadow from the eyelid, and make sure the top eyelid covers the top part of the eye.