Treadigraph almost had it! I've seen a small bolt (effectively the drawing pin) with its head held firmly against the skin being used as a pivot point. Then the individual roundel diameters were masked off with fine-line tape & paper and sprayed their respective colours in stages. Probably not the *proper* way but it worked.
I've heard of trammels and large sheets of tracing paper being used too. It must have been a real pain in the 30's when the red, white and blue roundels were ringed with yellow.