An oval cannot be drawn with a compass, and that is the first problem in making a mirror like this.
A circle has one centre, so a compass draws it in a single sweep. An ellipse has two, and its curvature changes continuously - tight at the ends, open along the sides. Sketch it freehand and the eye catches the error immediately, because an oval that is slightly wrong looks like an accident rather than a shape.
So it is set out the old way. Two pins are driven at the focal points, a loop of string is placed around them, and a pencil held inside the loop is run right around, kept taut. The string's length and the pin spacing between them decide the exact proportion of the oval - and by adjusting those two numbers, the shape can be made rounder or longer to suit the wall it is going on.
The frame is then built up in segments around that drawn line, each cut so its grain follows its own part of the curve, jointed where the ellipse is strongest and worked as one ring afterwards.
The flowers, the beading and the swags are all set out from the same drawing, so every ring stays concentric.