Tempo Reports

I'd start with the documentation here: https://docs.appian.com/suite/help/19.2/Tempo_Report_Design.html#creating-a-tempo-report

That's a broad question.  Eliot's response is, indeed, a good place to start.

But here are the broad steps:

1. Identify the scope and nature of the report you wish to build
2. Create the appropriate expressions or queries to retrieve the data you need
3. Build an interface that uses grids and charts to display the data as meaningfully as possible. You can use the report Builder or use one of the examples in the Interface Designer to speed up your build.
4. Build a 'Report' in Appian to expose the results to the user via Tempo or a Site 

If you are looking for more detail, and are finding the documentation confusing, post a more detailed question here for a more helpful response.