How does Napoleon trick the humans into thinking that Animal Farm is still successful and thriving?

Having expelled the hated human oppressor Mr. Jones from the farm, the animals are now firmly in charge. Manor Farm has become Animal Farm; the ideology of Animalism has triumphed at long last, finally realizing Old Major's dream. This is a good start, but it's not enough. Not only does Animalism need to be established, it must also be maintained. The animals need to show human beings that they can make it on their own, that they can establish a successful, thriving farm without any help or assistance from their erstwhile oppressors.
As winter sets in, however, things aren't going so well. The animals are working harder than ever, but thanks to Napoleon's incompetence, there's a chronic shortage of food. Just to make matters worse, the jerry-built windmill's now broken and so the animals will have to rebuild it, with all the crippling effort that that will entail. But Napoleon must keep up appearances at all costs; he must show the humans that Animalism is a living, breathing ideology that can inspire the animals to do better than humans.
On the domestic front, this means a massive increase in propaganda, with the ever-loyal Squealer giving long-winded speeches on the importance of heroic sacrifice. More ominously, the level of terror steps up. Snowball is being blamed for everything bad that happens on the farm. In this toxic environment of paranoia and delusion, animals are forced to make public confessions of assisting Snowball in acts of sabotage. They are then brutally executed for their alleged crimes.
In relation to foreign affairs (i.e., relations with humans) Napoleon enlists the help of a lawyer by the name of Whymper to act as an intermediary between himself and the outside world. When Whymper pays his weekly visits to the farm, Napoleon is anxious to make sure that everything seems on the up and up. Neither Whymper, nor anyone else from the outside world, must ever suspect that the Animalist utopia isn't working as well as it should.
Napoleon has betrayed the values of Animalism by agreeing to supply four hundred eggs a week to Whymper; and this at a time of chronic food shortages. But again, the facade of normality must be maintained at all costs. So a number of selected animals—mainly sheep—are instructed to remark casually within Mr. Whymper's earshot that their rations have increased, which of course is the exact opposite of what's really happened. Napoleon also gives an order that the empty bins in the storage shed must be filled with sand and then covered up with what little grain is left to make it look like there's a plentiful supply. Once again, this is a completely false picture of reality on the farm; there's actually a chronic shortage of grain, and the animals are staring starvation right in the face.