From a reference photo draw outline using a grid system - see my other video on this channel which explains how it is done using Photoshop
The lines are explained in the video mentioned before
The grisaille is done in a mix of lamp black and Underpainting White (By W&N - quicker drying). Use Liquin as a medium - again because it is quicker drying. Stays wet long enough to blend but dries over night
For the highlights. Mixes of TnWhite, Cad red, & sap green for nose, chin, and central forehead. Substitute Cd Red for Alz Crim for rest, with different tendencies.
For forehead, add yellow ochre or cad yellow and small amount of violet to the mix so it is at the yellow end of spectrum.
Break up the colour by having patches with a little more Az Crim, and other with more Sap Gn. More sap green/blue for temples of forehead. Orbit - add some Ult Blue/Violet.
Muzzle area tend towards green. Neck - tend towards violet. Lips - alz crim/yell ochre. Shadows of cheek and muzzle add some Ult Blue and brnt umber
For the darkest shadows. NO WHITE. Mixes of sap green, alz crim, violet, viridian and black. Sap green will lighten the mix for the lightest parts of the shadow.
Mixes of the others, except black will produce a near black. Use black in the mix only for the darkest dark - the left eye in this case. Leave the transition area between the highlights and the shadows till later.
Transitions forehead - Brnt Sienna and sap green. Nose and chin cad red and sap green.
Background -black and alz crim. Clothes - black/ult blue for darkest shadow. Highlights - Brnt Umb and Ult Blue and white.
Add some alz crim for reflected light. Add some hints of viridian to break up colour
Hair - brnt umb/ult blue/white in various combinations for a colder or warmer grey. Add black for the darker areas.
Moustache/eyebrows - plus some hints of brnt sienna.
Finish with some thin glazes of mixes of white, alz crim, cad red, sap green, violet, ult blue to various areas to alter colour/value slightly