The downloaded SVG is drawn at true size (one unit is one centimetre): print it at 100% and check the scale bar with a ruler. Printing this page fits the layout to the paper instead. Place the base of the gnomon on the marked point and turn the sheet so its north–south line points to true north (the direction of the shortest shadow of the day).
Measured from the foot of the gnomon: x towards true east (negative = west), y towards true north (negative = south). Hours are local solar time.
Measure at local noon, when the shadow is shortest and lies along the north–south line. Every noon shadow (except the two extremes at the solstices) happens on two dates a year, one as the Sun moves north and one as it moves south; you need to know which half of the year you are in.
The hour lines show local solar time. To get clock time, add the correction for the date: it combines your longitude's offset from the time-zone meridian and the equation of time. Daylight saving is not included.
The shanku (शङ्कु) — a vertical gnomon on level ground — is the basic instrument of the Indian siddhantas. The Surya Siddhanta's chapter on direction, place and time (Tripraśna) uses a 12-angula shanku to find the cardinal directions, the latitude, and the time from the length of its shadow.
For a Sun at hour angle H and declination δ, seen from latitude φ, the Sun's direction has east, north and up components E = −cos δ sin H, N = sin δ cos φ − cos δ cos H sin φ, U = sin δ sin φ + cos δ cos H cos φ. The tip of the shadow of a gnomon of height h lies at x = −h E / U, y = −h N / U. Holding δ fixed and letting H run through the day traces one date curve (a hyperbola in general, a straight line at the equinoxes); holding H fixed and letting δ run through the year gives a straight hour line.
Declinations come from the NOAA solar formulas for the current year; sankranti curves use the Sun's declination at each sidereal (Lahiri) sign entry. The figure assumes a sharp shadow tip from the centre of the Sun: in practice the Sun's disc (0.53° wide) blurs the tip, which limits readings to a few minutes of time and a couple of days in date.
At noon the shadow lies on the north–south line with length s = h tan z, where z is the Sun's zenith distance. Then δ = φ − z if the shadow points north and δ = φ + z if it points south (for a northern place), and the two dates with that noon declination are found from the Sun's yearly path.