The rules of this are simple, Take any movie or entire movie series, and then make a joke about what happens if you watch it backwards.
The format is "If you watch (movie or series) backwards then it's about....."
For example:
If you watch the Stars Wars trilogy (Ep. 4-6) backwards then it's about a kid who dooms the galaxy by rebuilding two Death Stars.
Or:
If you watch the Lord of the Rings trilogy backwards then it's about a kid who gets an awesome ring from a volcano but then has many many troubles walking home.
You can also make jokes about tiny things in the movie, like so:
If you watch A New Hope backwards, Greedo shoots first

.
That's the gist of it. Go ahead =).