Napoleon takes the puppies of Jessie and Bluebell very early in the book - section three. This shows that Napoleon right from the beginning was thinking about how to gain power. This is an important point to bear in mind, because he was more scheming than Snowball and any other animal for that matter. Here is the text:
It happened that Jessie and Bluebell had both whelped soon after the hay harvest, giving birth between them to nine sturdy puppies. As soon as they were weaned, Napoleon took them away from their mothers, saying that he would make himself responsible for their education. He took them up into a loft which could only be reached by a ladder from the harness-room, and there kept them in such seclusion that the rest of the farm soon forgot their existence.
Now as for your second question, there were many other ways in which Napoleon sought to gain and maintain power. First, he used rhetoric. He had Squealer who would always spin things to his favor. Also because Squealer was such a great speaker, he persuaded the animals that he was always right. In fact, this was one of Boxer's mottos.
Second, he limited the education to pigs so the other animal could not read. If knowledge is power, than he had all of it. This is why none of the animals could really create any type of revolution. The only one that could have was driven out - Snowball.
Finally, when Napoleon had things in place, he could do whatever he wanted. At the end of the book, he turned into a human and the animals did nothing, because they could do nothing - so complete was his power.