Here is a time-lapse animation of all the major stages:
The canvas is prepared with an oil-based ground of Gamblin Oil Painting Ground thinned to the consistency of milk. This "sizes" (water seals) the canvas and makes the surface less absorbent so that the paint will flow and the strokes will hold up their shape without sinking into the canvas weave. A warm semi-transparent tone is then applied to the canvas using a mix of raw umber (50%), yellow ochre (30%), ultramarine blue (10%) and alizarin crimson (10%), thinned with mineral spirits.
For a convenient self-portrait the canvas can be placed directly on a long mirror so that you have the option of painting in "sight-size" where the reflected image in the mirror is exactly equal in height and width to the drawing.
Stage 1: Block-In
The purpose of this stage is to lay in a rough drawing, capturing the major simplified shapes and their relative proportions using a warm, dark transparent wash that approximates the main shadow color. It's best to use large bristle brushes and create the initial drawing using straight lines and simplified shapes which will make it easier to evaluate the relative proportions.
Stage 2: Open Grisaille
Once the major proportions of the shadow shapes have been defined, bristle brushes are used with plenty of thinner to scrape away the under-tone and indicate the light areas by allowing the white of the ground to show through. This is called an "open" grisaille because white paint is not used.
Stage 3: Posterizing
The "average mid-tone" colors for the 3 or 4 largest shapes are mixed and laid in side by side so that their relationship can be compared on the basis of temperature (warmer / cooler) and value (darker / lighter). Typically this is the background, shadow shape, light fleshtone and clothing. These colors are laid in as flat "poster" shapes for easy comparison. Once they are all adjusted to work harmoniously they form the basis of the value structure of the painting. In mixing the "average" it is best to err on the side of darker middle values and low saturation. It's also important to keep a bit of clean thinner in the brush so that this layer will dry quickly. Pay attention to edges and "service the big shapes" by resisting the temptation to paint in smaller details. Maintain the simplest, biggest shapes possible.
Stage 4: Mid-Tone Mass-In
At this stage the brush is loaded with medium (50% stand oil, 50% thinner). The major shapes are broken down to the next level and the large mid-tone colors are massed in with large brushes starting with the shadow shapes and working into the mid-tones. The values are kept in the middle range for now.
At this stage the drawing needs refinement, and a more precise exploration of the anatomical shapes. I find it helpful to do a quick exploratory study of the major planes and shadow shapes, in keeping with the anatomical shapes identified by John Henry Vanderpoel in his classic "The Human Figure".
Using the drawing as a guide yields further refinement. The "architectural integrity" of the major planes is maintained, with well-defined edges that blend into each other without losing their borders. This way the sense of form and the structure of the head does not become "mushy" or "wobbly":
Stage 5: Modeling
At this stage more sensitive modeling is painted using a wider value range, crawling across the form from the shadows to the lights, but using "temperature" changes rather than exaggerated value changes to articulate the turning of the form.
Stage 6: Finish
Now working all around the painting rather than "crawling" across the form, the highlights, dark accents and saturated "broken color" passages are used to make important features like the nose extend out into space and draw attention to important moments such as the red of the lips, the high points of the chin, and center of interest in the model's right eye.
Here is a detail of these finishing passages. Note the "broken color" on the chin, and the transition from the "hot" pink of the right cheek to the "cooler" pink of the same value as the cheek turns: