You need an recorder app and a fairly beefy phone. I use AZ Screen Recorder, because it's free and does the job.
Keep in mind that video is huge, so don't keep it recording too long.
If you have an iphone, swipe up and press the record button. If you don't know how still, google 'how to record screen ios'. If you have an android phone I'm sure there are plenty of apps, if not one that comes on the phone that'll allow you to do so. Just do some googling man