First, find the App that you would like to add a Field to in App Studio. To do this click on the COGs menu and select App Studio which will open App Studio in a new browser tab. Once in App Studio select the App that you are adding the summary expression to. Lastly, click on reports folder in the App configuration to see a list of the reports in the App.
Assuming the app has the fields to be summarised in it already, the user can go straight to the List Report they what the summary expressions on. If you don't have a list report already built please see this article on how to configure one
In a List report, if the summary expression is defined (e.g. Sum) then the list report will show a subtotal at the top banner of the report. Summary Expression can only be found in App studio and has four functions (Average, Sum, Minimum and Maximum). In order for summary expressions to be used field needs to be number fields. To give a column its summary expression, you will need to go to the report field and select summary expression.
When a grouping filter has been applied - there will also be a breakdown of the summary expressions per grouping. See image below: