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Please tell me what model of (color) Canon ImageClass printer you need instructions for... but in general, the manual feed is most likely to be the tray or paper source that postcards would be printable from.
Please also tell me whether you are printing from Apple computer, or from a Windows PC. The settings differ considerably from one to the other. But in both cases, though they look very differently and are reached by different means, you need to set the printer's driver settings properly... and in Windows the 'driver settings' are reached by
Control Panel - Devices and Printers
and right-click on the icon for your Canon ImageClass printer, and select Properties. The Properties sheet for the printer is the driver settings, in Windows.