How do I load paper into the input tray?
Load the paper face-up with the top edge forward in the input tray, then adjust the paper guides until they are snug against the paper. When adding new paper, remove all existing paper from the tray and straighten the stack without fanning it to prevent multiple sheets from feeding through at once.
From the official HP LaserJet Pro MFP M128 Multifunction Printer manual, page 14
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