Procedure for Poster (classic)
Press [Original Settng].

Press [Original Size].
Specify the original size, and then press [OK].
Press [Original Orientation].
Select original orientation.

Press [OK] twice.
Press [Dup./Combine/Series].

Press [Series/Poster].

Press [Poster].

Select the number of sheets of paper you want to put in one poster.

Press [OK] twice.
Select the paper tray.