Plastic stencil, round hole the right size to match the British Roundel laid on the wing for correct position, a medium grey Pastel powdered onto some sandpaper, followed by the use of a cut down small brush to rub inside the stencil to create the round "painted" on circular pattern, onto the Matte paint surface...simple and precise.
Mk.IX shown...and Mk.V both now have their Gloss Coat...