Peel off white backing to expose sticky side and place permanently on the side of fermenter, do not try to remove once fitted.
Different colours will light up to show you the temperature range you are maintaining.
You will usually see 2 or 3 colours light up at a time. Either look for the "brightest" colour you see and then read the number beneath that for the temperature as they tend to read a little high, ie if 22 is really bright it's probably 20C.