Swipe down twice from the top of the screen to access the expanded Quick Settings. Press the pencil icon and drag the Screen Record tile up from the hidden tiles.
To start recording, swipe down to open the Quick Settings and find the newly added Screen Record tile. Tap it and confirm to start recording after a three-second countdown, shown on the right side of the notification bar.
To finish recording, swipe down to open the notification centre and tap Stop to save the recording. You can access the file in Google Photos> Library> Movies.