Catching Fire ends with Katniss, and the boy she is expected to love and marry, being lifted away from the Quarter Quell arena in a Capital hovercraft. A diary entry would focus on what follows this ending scene.
Therefore, your diary would include what you believe happens after Katniss is taken away in the hovercraft.
Suggestions to include in the diary entry are:
1. Where do you think Katniss went? What did she do when she arrived there?
2. Who was awaiting Katniss when she arrived? How did she react to those around her?
3. What was her first concern when arriving? What did she feel like she needed to do?
4. What are her plans for the immediate future? What are her plans for the future to come?
5. How does she feel about what happened to her? Does she feel like she has changed?