Full details of how the Reference Thermometer should be used can be viewed in the KM20REF User Manual.
Here is a brief overview and excerpt from that manual:
Make sure that both the food thermometer and your KM20REF have both been at ambient temperature (between +15°C and +25°C) for at least one hour.
Take a glass or plastic beaker that can safely hold at least 0.5 litres of fluid.
Mix cold water and if needed ice cubes made from tap water to achieve a mixture of water and/or ice (crushed ice is best) at a degree or so below your desired comparison temperature, say +4°C or +7°C as appropriate. You can use your reference thermometer for this.
0.5 litres of water in a plastic beaker at +7°C with an ambient temperature of 20°C exhibits a temperature rise of less than 0.1°C per minute.
Switch on both thermometers and insert both the probes into the water to a depth of at least 50mm (2 inches). Hold the probes so that their tips are very close together and use the probes to stir the water. The water should be stirred in the centre of the beaker away from the sides. The agitation rate should be approximately one rotation per second.
Once the KM20REF is displaying the required temperature of comparison and this reading has been stable for 30 secs the reading of the food thermometer can be taken and recorded.
If the reading from the food thermometer differs from the traceable reading of the KM20REF by ±0.5°C or more, the food thermometer and probe should be returned to the manufacturer for re-calibration.
Source: KM20REF User Manual 12116 Issue 10