Canon Color Bubble Jet BJC-5000 Series User Manual page 36

To remove curling from envelopes, hold the edges of the
envelopes diagonally and bend gently.
Make sure any curls are removed from the flap of each
envelope.
Insert the Slide a stack of envelopes into the sheet feeder until it
envelopes. stops.
Align the edge of the paper guide with the left edge of the
stack of envelopes.
Chapter 3 Paper Handling 3-13