A computer-implemented method performed by a data processing apparatus, for guiding installation of an opening sensor, the opening sensor comprising a magnetometer sensor and a magnet the method comprising: transmitting an indication to an installer of the opening sensor to open and close an entry point on which the opening sensor is installed; receiving (900, 1002) detected magnetic field strengths of a magnetic field created by the magnet of the opening sensor as detected by the magnetometer sensor of the opening sensor over a time period; determining that there has been no change in the strength of the magnetic field created by the magnet as detected by the magnetometer sensor based on the received detected magnetic field strengths over the time period and re-transmitting the indication to the installer of the opening sensor to open and close the entry point on which the opening sensor is installed; determining (902, 1004) that there has been a change in the strength of the magnetic field created by the magnet as detected by the magnetometer sensor based on the received detected magnetic field strengths during the time period; receiving (904, 1008) accelerometer data from an accelerometer disposed in the magnetometer sensor over the time period; determining (906, 1008) from the accelerometer data whether or not the magnetometer sensor was moving during the time period and if based on the accelerometer data it is determined that the magnetometer was not moving providing positive feedback to the installer of the opening sensor, and if based on the accelerometer data it is determined that the magnetometer was moving providing negative feedback to the installer of the opening sensor. transmitting an indication to an installer of the opening sensor to open and close an entry point on which the opening sensor is installed; receiving (900, 1002) detected magnetic field strengths of a magnetic field created by the magnet of the opening sensor as detected by the magnetometer sensor of the opening sensor over a time period; determining that there has been no change in the strength of the magnetic field created by the magnet as detected by the magnetometer sensor based on the received detected magnetic field strengths over the time period and re-transmitting the indication to the installer of the opening sensor to open and close the entry point on which the opening sensor is installed; determining (902, 1004) that there has been a change in the strength of the magnetic field created by the magnet as detected by the magnetometer sensor based on the received detected magnetic field strengths during the time period; receiving (904, 1008) accelerometer data from an accelerometer disposed in the magnetometer sensor over the time period; determining (906, 1008) from the accelerometer data whether or not the magnetometer sensor was moving during the time period and if based on the accelerometer data it is determined that the magnetometer was not moving providing positive feedback to the installer of the opening sensor, and if based on the accelerometer data it is determined that the magnetometer was moving providing negative feedback to the installer of the opening sensor.