Gnoblar_Herder has the right Idea.. Mostly you just need to sit down, plan it out and fail a few hundred times.. Thats how I did it, With Years of misshapen Ork heads..

Since its Good Practice for sculpting Faces in general, here is TINY smiling Nurgling Face =P
breaking down and doing the small details again and again is the only way to get better at it..

^ Ill be sculpting this Flat, Like I do a Number of bits.. We will be sculpting everything Forward of the ears slapping on the back of the skull after its had time to cure..
The round blob of Greenstuff has the ever so slightest peak where the nose will be..
Ill be using my hobby knife, a size Zero extra-firm Angel chisel Clay shaper(by Royal Sovereign), and a 0.3mm Mechanical pencil with the lead removed..

^ Using the Pencil Ill punch in the Eyes ((if you mess up or just dont like what youve done at ANY time, DOnt Save it! Mash it, and start over..))

^ Then I use the tip of the clay-shaper to pop up what will be the Nose..

^ My clean hobby knife cuts in the mouth

^ & Lastly I Turn back to my Clay-shaper working the face, giving it a touch of character..
This is pretty much How I sculpt All my faces.. After it has cured I can go back in and add finer details. Brow, cheeks, eyelids.. etc, etc...

*For Human sized Eyes use either a 0.5mm-0.7mm Mechanical pencil*

~Have Fun
You're all just Food..
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