Choose Format - Align - Centred (LibreOffice Writer, LibreOffice Calc).
        Choose Shape - Align - Centred (objects selected) (LibreOffice Draw).
        Open context menu - choose Align - Centred (objects selected) (LibreOffice Impress, LibreOffice Draw).
        On Align bar (LibreOffice Impress, LibreOffice Draw), click