The first thing you will need is to have a short video that shows your app in action.
There are different alternatives that depend on the mobile type you have.
For iPhone, the latest versions of iOS have a native screen video recorder that is very easy to use.
You can see the step by step of how it works in this link.
While Android does not have a native capturer, some apps allow you to do so. Here you can see the 10 Best Screen Recorder Apps for Android 2019.
You have to start recording and select the main features of the app you want to show.
It can be useful to record several clips or parts individually, or you can then cut the video into different pieces since you may want to show the functionalities at different times of the video.