How do you write a movie review?

I've watched 100's of movies from every genre, and I can talk about each movie with other people for hours, but when it comes to actually typing a review on the internet or writing it down I go blank and haven't a clue what to write. I'm a member on a movie forum, I like reading reviews but I was asked by the moderator to give reviews on some movies, even just a few lines, but I don't know what to write, any tips please? thanks