To write a diary entry, you would first want to reread the scene in question. In it, Mr. Sir takes Stanley to the Warden for stealing the sunflower seeds (even though Stanley is not to blame). Mr. Warden is very surprised when the Warden slaps his face. To make matters worse, she has something on her fingers that is so painful that it makes Mr. Sir fall to the floor, screaming and writhing in agony:

She stepped toward him and struck him across the face.

Mr. Sir stared at her. He had three long red marks slanting across the left side of his face. Stanley didn't know if the redness was caused by her nail polish or his blood. It took a moment for the venom to sink in. Suddenly, Mr. Sir screamed and clutched his face with both hands. He let himself fall over, rolling off the hearth and onto the rug.

Stanley wants to help Mr. Sir, who is in anguish, but he is too afraid of the Warden, so he leaves.

For the diary, you would have to imagine how Stanley processes his thoughts. Does he think the Warden is sadistic? Does he feel guilty that he was too afraid to help Mr. Sir? Does it feel worse to watch someone else suffer (to be a bystander) and not be able to do anything than to suffer oneself?

We can imagine that Stanley might feel upset and even mildly traumatized that he couldn't help a person in pain. He might berate himself for being a coward, and he might express his sympathy for Mr. Sir in writing since he could not in real life. He might also feel guilty for a second reason: if he been able to hide the seeds and bag, none of this would have happened. There is also the very strong possibility that he continues to feel very frightened of the Warden.