No matter how precise your electronic wind vane is, if the zero point of the wind vane's base is not oriented to north the readings will be in error.
There are some easy ways to find north. Once it gets dark look up into the sky and find the north star. When you have done that you are looking in the direction of true north.
Another aid can be our streets and roads. Many city streets are laid out in a north-south grid pattern. Just line the zero point of the anemometer so it points parallel to the north-south running streets.
The use of a compass to locate true north requires a little extra work. Magnet compass needles point to the magnetic north pole which is not at the same place as the geographic north pole. A correction must be applied to convert the magnetic north direction to the true north direction. The magnitude of the correction depends on where you are located. The following URL will take you to a U.S. government site that takes you through some very simple steps to convert magnetic compass readings to true direction readings for your location: www.ngdc.noaa.gov/geomag/faqgeom.shtml.
Once you have successfully aligned the anemometer to true north, your wind direction readings will report true wind directions within the limit of the errors of the anemometer.