If your Bluetooth device is paired and connected (check bluetooth settings)
Double tap the home button to go to the multitask bar, swipe right to see the controls. You should see the AirPlay icon (rectangle with triangle under it).
Tap this and choose your Bluetooth device. Go back to your movie app and watch.
The Airplay icon may appear in the movie app as well. If it does you can select the Bluetooth device from their instead. It depends on the app.