We first really see Squealer for who he is in Chapter 3, when the animals discover with some anger that the pigs have been taking all the milk and apples for themselves. At this stage, Squealer, a "brilliant talker", comes into his own as he explains to the rest of the animals that the pigs need the milk and apples because they are thinkers and the milk and apples help them think. As they are the best thinkers on the farm, it is obviously in everyone's interest to give the pigs all the milk and apples.
Note here and in the rest of the book Squealer establishes himself as playing a crucial role in creating and diseminating propaganda which gives the pigs an appearance of truth and common good with which they can hide their self-serving actions. Also key to think about is how what he says and what he does act as a perfect example of the psychological techniques the pigs use to control the farm whilst continuing to claim that this strictness is essential to avoid being taken over by man again. This of course echoes Orwell's view of how the initially laudable goals of the Russian Revolution very quickly were transformed into mere rhetoric as a means of controlling the people. Another strategy that Squealer adopts which is part of his success is that he feigns siding with the animals but always states that the "common good" is based on following the pigs' decrees. Quite funny for us reading it in some ways, but remember the chilling truth behind this allegory.