Once you have already created Report Templates either for specific categories or a multi-category Report Template you can then of course use them to generate an actual report for the current Revit Project.
Load all the required Revit Categories and ensure the column layouts for each Category tab is configured accordingly then click the Report button from the ribbon.
You can then select the required categories to be used in the Report you are about to generate.
Then you can select the required Report Template file (.repx file) the you have previously created and saved which will then launch the Report preview window as per below example.
From here you can then Save the resulting Report file or use any of the other options to either print a hard copy of the report or convert to other file formats etc.