What you would do is write the diary entry in first-person, giving your opinions of the the movie as you go along and making sure you write in past-tense. You could say that after watching Toy Story, you really liked how it depicted journeys (why you would write this in a diary, I have no idea), and give reasons and techniques used to support your statements.
For example, instead of writing, "Toy Story is a film that represents journeys through many different aspects", you could say, "I really liked the way Toy Story depicted journeys through many different aspects." You could then continue by giving examples and reinforcing that statement with how it is represented using techniques.
It really shouldn't be all that much different from the review format you've already written. Just a few simple tweaks here and there to make it look as if someone was writing a diary entry is sufficient. The teacher will be more focused about how techniques represent journeys.