What manipulation causes the animals to give Napoleon the credit for all of the good things accomplished on the farm?

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.