Fold the sheet. See the book.
Before a single copy is bound, a printer lays pages onto one flat sheet — some right-side up, some deliberately upside-down — so that when the sheet is folded and trimmed, every page falls in perfect reading order.
Place pages on the flat sheet, fold it, and watch. Get it wrong and a page comes out backwards or out of order — no harm done, just try the fold again. Some jobs hand you an already-printed sheet with a fault hidden in it. Find it before it goes to the customer.
Ten jobs, folio to master's proof.