First, you will want to Select Settings

Then select Control Center

Scroll down until you see Screen Recording and click the plus. You should now see it at the top.

Then you will want to go back to your Home screen and swipe down on the top right corner.

Then click on the Record Screen button and it will countdown from 3. 

Once it is recording, swipe up to return to the Home screen

Once you are finished recording, scroll down and click the Record button again

You can find all your Screen Recordings in you Photos app