Measure from the centre of the ellipse. x is towards true east (negative = west), y towards true north (negative = south). Find true north first, for example from the shadow at local noon.
A sundial shows sundial time. To match a clock, add the correction below for today's date (the equation of time). Daylight saving is not included.
The hour marks sit on an ellipse. Its east–west width is fixed, and its north–south height shrinks with latitude: height = width × sin(latitude). Each mark is placed at x = M sin H, y = M sin φ cos H, where M is half the width, φ the latitude and H the Sun's hour angle at that time.
Because the Sun is higher in summer, the person standing as the gnomon must move along the north–south line through the year, to y = M tan δ cos φ, where δ is the Sun's declination on that date. That is the date scale in the middle.
With clock time selected, the marks are shifted for your longitude, so the dial reads your time zone's standard time once the equation-of-time correction is added.