Draw your chosen subject onto bleed proof marker paper. If possible, use a light-box to trace the image (don't consider tracing as 'cheating'!!! Most professionals do it, it saves time and ensures an accurate likeness). Use an H pencil, nothing too soft that might smear. After drawing, it's an idea to gently go over the drawing with an eraser, to remove any heavy pencil lines that the marker ink might smear, 'dirtying' the image.
Using a deeper tone pen, start to block in the darker areas. Use the fine nib, it gives you more control. Remember, you can't paint on highlights (as you could with gouache), as marker ink is not opaque, so for now, note where the highlights are and leave them as white paper.
Select a lighter tone, and start to work in more coverage, again, observing where highlights will be. At the same time, you can block in very dark areas of shadow, so that you can see the image start to take shape.
Using even lighter pens, fill in the highlight areas, although leave extreme highlights as white paper (e.g. that point of light that you might find on the end of a nose). At this point, go over the work with a blender. This will 'fuse' the inks and smooth out distinctive lines where different ink tones have met. It will help blend in white highlight areas too. Don't worry if the tones now look a bit 'mottled'- once dry, you will then be working on top of these tones.
Once completely dry, start work again, deepening the dark areas, then adding to highlight areas again with light colour pens. As you would with a painting, the same principle of 'building up' layers of tone, applies. At this stage you can use an appropriately coloured pencil to sharpen lines around areas like the nose, nostrils, eyes, etc.
Once you are happy with the work, again allow to dry. Use the time to ink in things like clothes, or areas that simply require broad areas of flat colour (in this instance, the brim of the hat). Note that the inks will dry slightly lighter in appearance than when you first apply them, so more work will be needed.