Drag the middle line to set where the cut goes, and the edge lines to trim the scanner borders.
Cut
Outer margins
%
%
%
%
Rotate
0°
One angle for the page, measured and turned back. It is the simpler of the two models, and on most scans it is enough. If the pages still do not sit straight, turn this off and put the cut PDF through Book Scan Cleaner instead, where the angle can be set by hand page by page and a page that curves into the binding can be flattened.