Upon un-packaging the instrument, the barometer will need to be set to the local air pressure. This can easily be done by finding the latest pressure reading from the Met Office and using the observation site closest to where you live.
To set your barometer, carefully turn the adjusting screw on the back of the instrument to the right using a small screw-driver until the pointer value coincides with the actual atmospheric pressure reading you obtained.
Do not turn the barometer movement too far! Better precision can be achieved by Iightly tapping the barometer glass after adjustment has been made.