Assertion propaganda is basically when someone says something as though it is well-known fact, even if it may not be. This is done to make people feel like they cannot and should not dispute something they hear, and rather should accept it as indisputable fact.
That said, the pigs in Animal Farm use assertion as a tool forpropaganda masterfully throughout the book, to continuouslyweaken the rest of the animals from disagreeing with their leadership.
Look especially at the character Squealer for examples of this, and Napoleon uses it a number of times as well.
In Chapter VII, for example, Squealer tells the animals that they will no longer ever sing Old Major's original anthem, Beasts of England, which was used to inspire their revolution in Chapter I, as it is no longer needed. Look at the language he uses to make them feel like the information he is telling them is already obvious, they should already have realized it, and there is no possible argument against it:
"In Beasts of England we expressed our longing for a better society in days to come. But that society has now been established. Clearly this song has no longer any purpose."
He then goes on to introduce a new song that is solely about praising Napoleon. He is able to do this without the animals questioning it, even though they want to. It says, "The animals were taken aback," but none of them argue because of the way Squealer used assertion.
This can be found in numerous other places throughout the text. Look especially through Chapter 7, as well as the final few chapters, when things really start to change drastically on the farm. Think of all the times Napoleon and Squealer say things like, "Surely, you don't want Jones back," or "Surely life is better now than it was under Jones." These are the examples you're looking for. Ones where Squealer and Napoleon are speaking as though their words are the absolute, obvious truth and cannot be debated.
I've attached two links below that will also help you identify more examples of assertion propaganda in the book.