One thing that you need to pay attention to before writing this diary entry is you need to determine how far into the book your supposed entry is being made. If your diary entry is taking place only after Stanley has been at Camp Green Lake for a couple of days, then you aren't going to have a lot of textual evidence to draw on. The prompt says your diary entry is being written after having been at Camp Green Lake for "so many days." That seems to indicate to me that Stanley has been there quite a while.

This is good because it allows you to discuss Mr. Pendanski. Mr. Sir is easy to describe. He's a vindictive monster. He absolutely doesn't like the boys, as indicated by letting Stanley know that he isn't worth the bullet. In fact, he doesn't even care about the lives of the boys, and solid evidence for your letter is the fact that he deprived Stanley of water for over a week. Mr. Pendanski is a more curious character.

Early in Stanley's experiences, Mr. Pendanski appears somewhat nice and caring for the boys; however, Stanley eventually comes to see that Mr. Pendanski might be just as cruel as Mr. Sir. Stanley could write about how Mr. Pendanski especially hates and antagonizes Zero. It's Mr. Pendanski who deletes Zero's files, which is a clear indication that he cares more for his own interests than the interests of the boys.