HP Deskjet 6122 Color Inkjet Printer - Manual (Page 21)

load paper in the tray
If you are using a paper size different than the paper size last used in the 250-Sheet
Plain Paper Tray, then you must set the paper size .
Follow these steps to load paper:
1. If necessary, remove the lower tray from the base.
2. Insert the paper into the lower tray.
3. Insert the lower tray into the base until it snaps into place.