Paper tray
When printing on 2-sides, load preprinted paper with the side to be printed as the first page facing up.
Paper cassette
When printing on 2-sides, load preprinted paper with the side to be printed as the first page facing down.
Select Preprinted as the paper type on the paper settings screen displayed on the control panel.
You can also display the paper settings screen by selecting 
on the control panel.
Check the printer driver settings.