Canon MultiPASS C50 Software User's Manual page 25

Press all the way around the envelopes to remove any curls and
expel air from inside the envelopes. Make sure any curls are removed
from the flap of each envelope.
3. Insert the envelopes into the C50. Slide a stack of envelopes into the
sheet feeder until it stops. Align the edge of the paper guide with the
left edge of the stack of envelopes.
You are now ready to begin printing your envelopes.
3-4 Printing with the C50 Chapter 3