To use the new screen recording feature in iOS 11, you'll first need to add the feature's button to Control Center.
Open Settings > Control Center > Customize Controls then tap the green plus button next to Screen Recording.
Record without sound
With the shortcut added to Control Center, swipe up from the bottom of your iOS device's screen and tap the Screen Recording button. The button will begin counting down from 3, giving you time to get to the screen you want to record. The status bar at the top of your phone's display will turn red when it's actively recording. Tap the red bar to instantly stop recording.
Narrate as you record
If you have a 3D Touch-enabled iOS device, hard-press on the Screen Recording shortcut in Control Center. If you lack 3D Touch, a long-press will do.
A button to turn Microphone Audio On or Off will show up just beneath the record button. Tap the button to turn audio recording on, then tap Start Recording. The microphone button will turn red when activated as a reminder that audio will be captured. Now iOS 11 will record using your device's microphone along with whatever is on the screen.
View and share
Once you stop recording, the video is saved to your phone's Camera Roll on your device. Open the Photos app to find it. You can then view, edit and share the recording as you would a video you recorded using the Camera app.