The first step is to paint a reclaimed canvas white.
The second step is to divide your canvas up in sections, rule up lines so that your triangles run along a straight line.
Draw your triangles onto the canvas with a grey lead.
Outline with a narrow felt tip pen.
Then start filling in your triangles with a thicker marker. Randomly leaving a small selection free from fill and lightly fill in others.
And here is the finished product. It looks very similar to the original displayed at the top of this post.