For most applications (looking for thermal leaks in your home), start with the slider set to 5ºF (3ºC). This means that the green spot of light will change to red or blue if the surface temperature changes by more than 5ºF (3ºC).
If you don't see any color change as you scan the surface, try the 1ºF (0.5ºC) setting. This might be necessary if the outside temperature is not much different from the inside temperature of your home.
If the color changes immediately as you scan the surface, try the 10ºF (5.5ºC) setting. This might be necessary if the outside temperature is much colder or hotter than the inside temperature of your home.
If you just want to measure the temperature of a surface and you don't need to find hotter or colder areas, slide the switch to the top position, where the blue and red circles are crossed out (indicated by the X). When the threshold switch is in this position, the colored spot will stay green, regardless of the temperature.