How do the pigs continually strip Snowball's honor?

The pigs, particularly Napoleon and Squealer, continually strip Snowball's honor away by "revising" history. We as readers, for example, are privy to a detailed account of the Battle of the Cowshed, and know that Snowball not only anticipated the attack and studied how to repel it, he also fought bravely. As the text explains:

Snowball now gave the signal for the charge. He himself dashed straight for Jones. Jones saw him coming, raised his gun and fired. The pellets scored bloody streaks along Snowball's back, and a sheep dropped dead. Without halting for an instant, Snowball flung his fifteen stone against Jones's legs. Jones was hurled into a pile of dung and his gun flew out of his hands.

However, later on, when someone mentions that Snowball fought bravely in that battle, Squealer dismisses Snowball's actions and states that loyalty and obedience matter more. After more time passes, Squealer changes the story even more, lying utterly and turning Snowball's intelligence and courage into treachery:

Snowball was in league with Jones from the very start! He was Jones's secret agent all the time. It has all been proved by documents which he left behind him and which we have only just discovered. . . . Did we not see for ourselves how he attempted—fortunately without success—to get us defeated and destroyed at the Battle of the Cowshed?"

When Boxer protests that what they saw was Snowball bloody and wounded at the battle, Squealer retorts that this was simply part of the plot—the bullet only grazed him. Again lying blatantly, Squealer says that Snowball ran away and that Napoleon led the charge against Farmer Jones, biting him in the leg. This is the exact opposite of what really happened, but by now the animals are confused.
Orwell shows how totalitarian states use propaganda to spread lies and pin their leaders' own wrongdoings on innocent parties.

After Napoleon runs Snowball off the farm, Snowball's honor is continually stripped away. In the aftermath of his expulsion, for example, Squealer says that Snowball's role in the Battle of the Cowshed was "exaggerated" and that his desire to let the animals make decisions for themselves was wrong and not in their best interest. In addition, the shed where Snowball drew up plans for the windmill is closed.
In chapter 6, when the windmill is destroyed, the pigs immediately blame Snowball. They portray him as an enemy who works with the humans in order to bring ruin on the farm. In fact, you'll notice that every time something goes wrong on the farm, Snowball gets the blame.
In this same chapter, Snowball is stripped of his title, "Animal Hero First Class," and Napoleon sentences him to death, should he ever return to the farm.
Through these actions, the pigs successfully blacken Snowball's reputation and convince the animals that Napoleon was right to expel him from the farm.