how did he lead the other animals in this assault ?

Okay, this happens in chapter 4. Snowball has already trained the animals for war, he had studied the tactics of Julius Caesar. The smaller animals attack the men and then pretend to retreat into the yard in defeat. Once the men follow, the larger animals ambush them. Mr. Jones kills one sheep and wounds Snowball several times with his gun, but the animals manage to overpower the humans. Boxer is thought to have killed a stable-lad, which upsets the stalwart horse. But it turns out that the boy is only injured, and he flees with the other men. They invent a military honor called “Animal Hero, First Class,” which they bestow upon Snowball and Boxer.