This is how you record video of what happens on the screen in iOS 11.
1. Swipe up from the bottom edge of the screen to open control center.
2. Tap the screen recording button (the one with a white circled dot) to start recording.
A timer starts counting down from 3 to give you time to prepare or exit control center.
3. Once you are done recording you can tap the red line along the top or open control center and tap the blinking screen recording button.