Draw your chosen subject onto bleed proof marker paper. If possible, use a lightbox 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 which the marker ink might smear and 'dirty' the image
Using a deeper tone pen, start to block in the darker areas of the image. 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 (eg 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 (leave the work for about half an hour), start work again, deepening the dark areas, 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 etc, or areas that simply require broad areas of flat colour (in this instance, the brim of the hat). Note that you must allow the ink to dry, and then go back to the work...the inks will eventually dry slightly lighter in appearance than when you first apply them, so more work will be needed.
Again, once dry, look at the image and decide where it could use more work, either going over areas with light colour pens to smooth over areas that look a bit rough, or deepening shadow areas with darker tones. Finally, spray mount the paper onto thin white card. Once done, you can work in some fine detail (eg creases around the eyes, eyebrows, hair etc) with colour pencil. You'll even be able to add some small highlights with a WHITE colour pencil. Pinpoints of 'light' (eg the white points of light in the eyes) can be added with a white ink technical drawing pen, or white paint applied with a fine paintbrush