As you have not mentioned anything, about what you have tried already, I assume that you have not tried anything. So, I am giving you the steps to load the envelope.
You can also open the link below to get these same steps with illustrations.
Loading Paper and Envelopes in the Rear Feed Slot
You can load one envelope or one sheet of paper or card stock in the rear feed slot.
Note: If you are using legal-size or longer paper, do not open the paper stopper.
Always follow these paper loading guidelines:
Note: You can load a sheet of binder paper with holes on one of the long edges, but not for automatic 2-sided printing.
Loading Envelopes in the Cassette
You can print on plain paper envelopes in this size: No. 10 (4.1 × 9.5 inches [105 × 241 mm]). If your product has two paper cassettes, you can load No. 10 size envelopes in Cassette 1 only.
Note: Make sure the envelopes are loaded against the edge guide and not sticking out from the end of the cassette.
Caution:To prevent paper feeding problems, make sure you do not load too many envelopes in the cassette.
Note: The envelopes may slide forward slightly as you insert the cassette. This is normal.