The masons who built Chartres and Notre-Dame drew their rose windows full size on a plaster tracing floor, with nothing but a compass, a straightedge and a cord. Build one the same way, one construction at a time: divide the circle, raise the Gothic arches, set the oculi and the foils, and the symmetry repeats each step around the center. Then watch the whole window being built.
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Compass work
The equilateral arch
Open the compass to the width of an opening and, from each side, strike an arc through the other: the two arcs meet at the point of the Gothic arch. Its two halves and its width make an equilateral triangle. Split the opening in two and do it again with half the width: one more stroke of the compass, from the same point, finds the circle that touches all three arches. That is the core of the "geometric" tracery of the 13th century.
Foils and cusps
A foil is a lobe, like a leaf (folium): three make a trefoil, four a quatrefoil, and so on. Here they are circles set inside a circle, touching it from inside and cutting each other. The points where the lobes meet are the cusps. A rose is just these few moves repeated around a center: circles divided, arches raised, foils set in the circles. Sacred geometry made into stone and light.