Watch the movie twice. The first time, just watch it. The second time, take notes on whatever you feel a reader of your review could benefit from.
Begin the review by going over the plot. Do not give anything away, but basically write what would appear on the back of a DVD case.
Move on to discuss the technical aspects. In what style was it shot? Did it remind you of another director? Was the editing seamless? How was the timing?
Avoid using the words - my, I, and opinion. Everyone reading a review knows YOU wrote it and that it is YOUR opinion. In doing so, though, be sure to make a reader conscious of the fact that your opinion does count because of movie knowledge and production knowledge.
Critique the acting. Was it over the top or just right? Did you relate to the characters? Did you find yourself spellbound at the performances?
Compare this film to other works of the director. Has he improved from prior efforts? Has he utilized similar themes, motifs, and styles? Or is he branching out?
Rate the movie. 1-4 stars, 0-10 scale. Keep in mind that this is what most readers will use to gauge interest. The average movie-goer has no idea what cinema verite means, but they do know what 1 and 1/2 stars means. Overall, be fair, but honest.