RottenTomatoes Is Your Friend
On Give a Good Review to a Bad Movie Day, think of the worst movie you've ever seen. Maybe it wasn't really all that bad because you have impeccable taste when it comes to choosing movies to see. Just select the worst of the best.
Then try to convince someone else to see it. You could do this online or in person. There are movie review sites where you could submit your opinions. If you have a blog of your own, you could post it there.
If you don't have a blog but want to start one, I can help you for a reasonable fee. (This part is serious.)
I don't watch all that many movies. My times for viewing them are few with a lot of time in between, so it's hard to remember what I've even seen. My worst is probably some made-for-TV flick that I didn't even know the title of while watching it.
If you watch a lot of movies, the worst one you ever saw probably starts with "Attack of the Killer...", doesn't it?
Maybe you should watch it again before composing your review. I'll get the popcorn.