Note that the front single-sheet paper feed is suitable only for sheets of A4/A-size or larger.
...or read this explanation:
Place the paper in the output tray, aligning it on the right of the output tray, with the side that is to be printed on down.
Push the paper into the printer, pushing it in as far as it will go, while maintaining its alignment to the right.
Press the OK key (the upper right key) on the front panel.
The printer will check the alignment of the paper, and if it is misaligned, it will eject the paper; in that case you will have to reload the paper.
Note that you can press the OK button to load paper before sending a job to the printer; you should also press this button if the paper fails to load, to tell the printer to retry loading it.
Select MANUAL FEED as the media source from the printer driver.