How was the idea of Animalism described by Old Major in Animal Farm?

In Animal Farm, Animalism is not developed until after Old Major's death. His speech, however, given a few days before his death, is instrumental in defining many of the principles of Animalism, and this is evident in a number of ways.

First of all, Old Major develops the idea that Man is the enemy. We see this through the following lines from his speech:

Because nearly the whole of the produce from our labour...

In Animal Farm, Animalism is not developed until after Old Major's death. His speech, however, given a few days before his death, is instrumental in defining many of the principles of Animalism, and this is evident in a number of ways.

First of all, Old Major develops the idea that Man is the enemy. We see this through the following lines from his speech:

Because nearly the whole of the produce from our labour is stolen from us by human beings. There, comrades, is the answer to all our problems. It is summed up in single word - Man. Man is the only real enemy we have.

Once the animals realise that Man is the cause of their problems, they use Animalism to protect themselves accordingly. This is best represented in the first of the animal's commandments: "Whatever goes upon two legs is an enemy."

Secondly, Old Major convinces the animals to unite in the struggle against Man. He develops the principle of equality between all animals, from pigs and dogs to mice and rabbits, and urges them to work together to overcome Man. We see the influence of this idea in the second and seventh principles of Animalism: "'Whatever goes upon four legs, or has wings, is a friend,"  and "All animals are equal." In fact, Old Major utters these phrases to the animals and they are copied in their exact form. 

Finally, Old Major identifies a number of human vices which he urges the animals to avoid, at all cost. These include living in a house, drinking alcohol and sleeping in a bed. For Old Major, animals who seek to resemble Man will become tyrannical and selfish, and therefore these habits must never become commonplace. Again, the animals are profoundly influenced by these ideas and adopt them directly into the seven commandments, specifically in the second, third and fourth.

(Please see the reference link provided for a full-text of the commandments).