To open the Report editor, go to Report -> Report editor in the menu bar.

For better understanding, we divide the window into "1. Report editor: selection" and "2. Report editor: queries".

Pull subgroups from the left in the middle pane to build a query.

In the right pane the fields will appear for each chosen group:

there you can select which fields should appear in the result.

To define conditions on a certain field, double click it's label.