In composing a journal entry or a series of journal entries on Napoleon, I think that it is important to emphasize that much of it is going to be dependent on when you are writing as Napoleon. For example, the Napoleon in chapter 2 is going to be different than the Napoleon in chapter 7. In this, I think that you can delve into the characterization of Napoleon and reflect his inner thoughts.
For example, a diary entry about Napoleon in chapter 2 might focus on how he believes in Old Major's vision. It might also suggest how excited he is to bring out Old Major's vision of Animalism into fruition. It might also talk about how he is working the farm to speak with each animal about embracing a world in which the animals are in control of their own political and social destinies. I think that there is the idea in this diary entry of Napoleon that the essence of change lies in how the animals perceive themselves and their relationship with human beings.
A diary entry of the Napoleon in chapter 7 is probably more pragmatic, less idealistic. In this, Napoleon would come across as more brutal of a leader. He would talk about the public executions and perhaps there is an explanation of why he did what he did and how there was really no choice within it. In this vision, Napoleon is more hard bitten and practical about his role as leader of the farm. In these diary entries, much of the reflection that you would give Napoleon is reflective of the particular point in the text.