I have never printed on anything smaller than 8.5x11 paper so I don't have any experience with 4x6 or 5x7 sheets. According to the 3880 User's Guide you simply load the paper in the Auto Sheet Feeder (top slot) like any other paper size.
To load fine art matte papers and other thick stock you must attach the Rear Sheet Guide onto the back of the printer, close the top slot cover, and manually feed the paper into the rear feed slot. Push it down with slight pressure and the printer will recognize the paper and finish the feeding process. Look at the LCD for a "ready" message.
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