Step 3
I used the same process to draw the aeroplane from a reference photo. I then positioned the heads and two planes appropriately, the planes rotated to point in the same direction as the figures' gaze. I added lines to the planes to add to the sense of motion towards the top right. Then I hid these to work on the rest of the composition.
I also selected a colour scheme, picking the orange flesh colours from an old US military poster and using warm shades of yellow and brown to match.
Step 4
The next step is to fill this line art with gradients so you can really feel the geometry. Orient all the gradients so that the dark parts correspond to shadows. Note that the cheekbones are lighter than the area surrounding where the eyes would be.
Step 5
To add grain, import the image into Photoshop and go to Filter > Noise > Add Noise... Select parameters as you like, though your choices will depend on the size of the file you're working on. If you work at 300dpi you'll need more grain than if you work on a small piece.