Use the official smartthings home monitor (STHM) feature in the new V3 app, or the official smart home monitor (SHM) feature in the classic V2 app. 
Official instructions for both on the following page. Make sure you have clicked the tab that matches the version of the app that you have.
And just in case you don’t know which version of the mobile app you are using:
FAQ: how to tell which version of the SmartThings mobile app that you have