Manual duplex printing asks you to feed printed sheets through the printer again. The difficult part is preserving the right face, leading edge and stack order. Copying a loading diagram for a different printer can put the second page on the first page's face.

Map one sheet first

Read the manual for your exact printer and its input tray. HP's instructions illustrate different reload directions for top-loading and bottom-loading designs. Treat the displayed reinsertion prompt as specific to the selected driver and printer, not as a universal rule.

Use a harmless four-page test document with a large page number and an arrow labelled TOP on each page. For an initial one-sheet feed check, put a small pencil mark near an edge of plain paper and note its position in the input tray. The resulting print reveals which face and edge the printer used.

Keep that observation beside the printer while you perform the short duplex trial. Do not apply this experiment to unsupported media or put adhesive markers through the paper path.

Keep the second pass controlled

When the driver pauses, preserve the output order. Follow its instruction for reloading the stack and continuing. Where instructed, remove unused blank paper first so the second pass cannot pull an unexpected sheet. Do not mix a separate print job into the paused trial.

After completion, read the four numbered pages in sequence. Check that each back belongs to its front, all page tops are correct and no blank or duplicated page has appeared. A correct first sheet alone does not prove the stack order.

Only increase the batch size after the trial passes. Keep the successful loading note with the tray name, printer model and driver used. If feeding becomes unreliable, stop the batch instead of repeatedly sending the same pages and losing track of which side has already printed.

Sources: HP manual duplex instructions.