Canon iP4700 SERIES On-Screen Manual page 36

Press down on all four corners and edges of the envelopes to flatten them.
If the envelopes are curled, hold the opposite corners and gently twist them in the
opposite direction.
If the corner of the envelope flap is folded, flatten it.
Use a pen to press the leading edge in the inserting direction flat and sharpen the
crease.
The figures above show a side view of the leading edge of the envelope.
Important
The envelopes may jam in the printer if they are not flat or the edges are not aligned. Make
sure that no curl or puff exceeds 0.1 inches / 3 mm.
2.
Load envelopes.
Open the Paper Support, and pull it out.
(1)
See in "Loading Photo Paper" of "Loading Paper in the Rear Tray".
Load paper.
Open the Paper Output Tray gently, and extend the Output Tray Extension to
(2)
open.
in "Loading Photo Paper" of "Loading Paper in the Rear Tray".
See
Load paper.
(3) Slide the Paper Guides (A) to open them, and load the envelopes in the center of
the Rear Tray WITH THE ADDRESS SIDE FACING YOU.
The folded flap of the envelope will be faced down on the left side.
Up to 10 envelopes can be loaded at once.
(4) Slide the Paper Guides (A) to align them with both sides of the envelopes.
Do not slide the Paper Guides too hard. The envelopes may not be fed properly.