Hello, nice to meet you. Well I ain't a professional at pixel art, in fact this is my first post, however I have been observing for quite some time.
I tried what I thought it should look like. Now that I look at it I think the hat should have more of a slight curve in the lighting.
Anyway:
Alright for the top hat I adjusted the light source slightly so it wasn't a straight line down and gave it some dithering.
On the moustache I altered it completely thinking of the light source comming from the left. I imagined how it would bulge in and out and lighted the sections I imained would catch the light from a said angle.
I did the same with the nose as well as altered the glasses.
Before you take example of what I did you should probably wait for someone else to comment on what I did as I may be wrong.