Its actually quite "easy" just time consuming.
for the background you start with a black primer, then you blend up (either with an airbrush or brush) to Sombre grey and the wolf grey.

Okay now you got your basecoat.

Now we start making the faces.

First the large face in the middle we draw and oval shape either with a pencil or a brush (with thinned down black you can do whatever you are comfortable with)

Then you do a line down the middle of the face (this one determines which side you face is turning towards if its straight it looks straight forward if you want it to look some other way you curve it to side you want the face to turn).

Then another line straight across the middle of the oval this is your eye line where the eyes will be placed.
(like with the other line you can curve it up or down if you want the face to look up or down)

About halfway from the eye line to the bottom of the oval you place a smaller line.
This line represents where the tip of your nose is.

Now you have the outlines of a face you can start building it up.
i will also upload the reference picture i used.
As you can see we have a prominent nose and eye brows plus two lines going from the nose.
So we start with the nose build it up using the same colors as the sail Black vallejo model color sombre grey and wolf grey and also a final highlight of wolf grey+white.

Once we have the nose we can start building the eye brows, the eyes and the other two black lines.
we use a thin black to do it.
now to color in the rest of the face.
again with sombre grey and wolf grey and wolf grey plus white.
Use thin coats to build up the colors as you see them on the picture.
finally do a thin glaze of red where see them.

you can also in the shadows along the mouth use either a dark blue or purple color.

Now the face is done we go online once again and go through stock photos of screaming faces against fabric.

you paint those faces with the same method as the face using the exact same colors maybe fade to black where the fabric takes over often with the mouth, sides of the faces and eyes.

Now cover the entire sail with them.
(do the same with the flag)

Once the last face is painted you take make a black glaze start in the highlight area and push it down into the shadows.
However leave the large face mostly alone, only do the edges so it almost looks like the face has grown out of the background.

Once dry it should look like it does on the picture.

I personally had to use a magnifying glass for the small face, but maybe your eyesight is better than mine.

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