Brother HL-3400CN Series User Manual page 66

USER'S GUIDE
3–12
Using Envelopes
You will need to attach the Envelope Adapter when you want to print
envelopes.
1. Pull out the Media Cassette and remove the Media Cassette Cover.
2. Attach the Envelope Adapter to the Media Cassette. Secure the
hooks of the Envelope adapter into the Media Cassette.
Fig. 3-3 Attach the Envelope Adapter
3. Adjust the Envelope Guide to the size of the envelopes you want to
load.
Fig. 3-4 Adjust the Envelope Guide