I need help writing a single diary entry from the perspective of a citizen of Oceania.

I am presuming that by a "citizen" you mean a Party member in the world of 1984.
To be descriptive means to use the five senses of sight, hearing, smell, taste, and taste to build a sensory awareness of what is going on. Orwell provides a good example of how to do this in the opening chapter of the novel, as he follows Winston through coming home from work and mounting the steps to his apartment.
To describe, you want to imagine you are filming a movie of the citizen's day, highlighting the most important points.
From what we know, a citizen is likely to be uncomfortable for most of the day. You might want to emphasize the lack of heat, the poor food, a tiny apartment, and the broken elevators. What does the person eat? What is the morning exercises routine that they experience through the telescreen? Would the citizen have any friends or would that be too risky? What would their job be like? You might describe a Two Minutes Hate session. Does the person love Big Brother? Does the citizen like their life?
Orwell mentions items like posters of Big Brother everywhere, Victory gin, rationing, and possibly enemy rockets hitting a target—all of these are everyday items you might want to mention.