What manipulation causes the animals to give Napoleon the credit for all of the good things accomplished on the farm?
Squealer becomes Napoleon's spokesperson. Squealer has written poems and songs before his rise in position. He blatantly changes the history of the farm simply by rewriting it. The commandments change according to Napoleon's own actions, and most of the animals are oblivious to the manipulations of Squealer.
Squealer explains things to put Napoleon in the best light. After Snowball is chased off the farm and Napoleon announces they will build the windmill, Squealer informs the crowd that Napoleon was never against the windmill project. He blatantly alters the facts to make a new history.
Squealer is able to retell the history of the battle in which Snowball was previously credited with the victory ,to show that it was actually Napoleon's mastery that led to their victory. When Napoleon dictates that he will now be in charge of all the decisions, Squealer states that Napoleon is doing it for the animal's own good. He tells them Napoleon does not want them to be responsible for a bad situation that would put Jones back in charge.
When Napoleon breaks the commandment on alcohol consumption, Squealer alters the commandment to suit Napoleon's purposes.
In the end, the seven commandments are reduced into one. Squealer is the ultimate tool for Napoleon, a very gifted "spin doctor".
The animals simply refer to Napoleon's leadership as the reason for good things happening. The lines from the text are as follows:
"It had become usual to give Napoleon the credit for every successful achievement and every stoke of good fortune. You would often hear one hen remark to another, "Under the guidance of our Leader, Comrade Napoleon, I have laid five eggs in six days"; or two cows, enjoying a drink at the pool, would exclaim, "Thanks to the leadership of Comrade Napoleon, how excellent this water tastes!""
Then, of course, Squealer has painted Napoleon's portrait on the wall across from the commandments in the barn next to the words of a poem composed by Minimus called Comrade Napoleon.
Another more minor example would be the animals' new tendancy to make up new titles for Napoleon like Father of All Animals, Terror of Mankind, Protector of the Sheep-fold, Decklings' Friend, etc. It's tough to not believe the greatness of one individual when he's presented in that much glory.