Take notes on the movie as you watch it. Before you sit down to watch a film, get out a notepad or a laptop to take notes. Movies are long, and you can easily forget details or major plot points. Taking notes allows you to jot down little things you can return to later.
Make a note every time something sticks out to you, whether it's good or bad. This could be costuming, makeup, set design, music, etc. Think about how this detail relates to the rest of the movie and what it means in the context of your review.
Take note of patterns you begin to notice as the movie unfolds.