Take the block of pages and insert into your clamp. Make sure that the spacers are in place underneath the clamp. Also ensure that the pages are square up against the abutment.
Notice that the clamp is about 5 mm above the work surface. This will mean that 5 mm of paper will stand proud of the clamp when it is turned up the other way. You might want a different thickness of spacers to select your preferred amount by which the sheets stick out. Pinch up the wing nuts, then turn the clamp over and do a quick check. When the block is nicely square and the spine edges are all flush and even, tighten the nuts evenly.