Canon MultiPASS C3500 User Manual page 61

Insert the Slide a stack of envelopes into the sheet feeder until it
stops.
envelopes.
Align the edge of the paper guide with the left edge of the
stack of envelopes.
Insert the envelopes with the print side up.
Feed the top of the envelope (return address portion)
into the sheet feeder first.
Make sure the back flap is toward the sheet feeder.
Now you can start your print operation.
Chapter 2 Printing With the MultiPASS C3500 2-27