Canon Pixma iP90 User Manual page 96

Set the Paper Thickness Lever (gray) to the left.
Note If the envelopes do not feed properly, even after removing curl, clean the Paper Feed Roller.
See Paper Does Not Feed Properly or Paper Jams
Also, if you decrease the number of paper loaded, it may be fed successfully.
Printer Driver Settings
Media Type: Envelope
Page Size: Select either DL Env. or #10 Env. according to the envelope size you are using.
Orientation: Landscape
Handling of Printed Output
Do not stack more than 5 sheets in the Paper Output Slot Cover.
Printing Area
See Printing Area