Step 1: Find where true north is. This is best done on a clear night when you can identify the Big Dipper and the North Star.
Step 2: Shove a stick in the ground and line it up with the North Star. This may require laying on the ground and aiming the stick like you would aim a rifle. Now you have your
gnomon.
Step 3: Next day, when the sun is out, calibrate your gnomon by observing its shadow. At
local solar noon, the shadow of the stick will point to true north and the shadow will be the shortest of the day.
Step 4: Read up on
analemmas so you can correct for the eccentricity of the Earth's orbit that makes the sun's position relative to the gnomon vary from month to month.