You'll want to start with a list report containing the fields you want to expose as the fields of the data source. For example, if you wanted to create a field containing calculations that tied back to an Opportunity subject in your data, you might add the Opportunity ID (to join the data back), filtered fields of Won Opportunities, Pending Opportunities, and Lost Opportunities, and a calculation of Average Opportunity Time to Close. Save that report.
Next, go to the Connections page and choose the "Report Source" connection. You'll be prompted to pick the report you want to use to create the connection. Choose the calculation report you created earlier and give the data source itself a name (we typically use the report name for consistency, but you don't have to). Click on Create and you'll have your report data source.
From here, you can either use the report source alone, you can combine it with other sources, or you can fold it back into the source it came from. For #1 and #2, it'll be like anything else with data sources. For #3, the way to do it is to go to your original data source, click on Configure the Data Source, and click into the "Additional Sources" section. Add your report data source and any necessary joins and it'll be available to you just like any other data source in that combined source.