That's much harder to break down in a tutorial and better described by more extensive works of art literature as it's more of a combination of understanding anatomy, form and value. Painting the greyscale itself took like 15 minutes.. I do it all the time almost every day so it's kind of an automatic thing now. Quick scribble to plan out the image, block-in, basic values and foundation.
I'm not super verbose about this stuff so I'm not the best person to go into raw art foundations.