Follow these steps to load envelopes:

Open the paper support, then slide out the extension, and then open the feeder guard.

Lower the output tray, then slide out the extension.

Slide the left edge guide to the left.

Load envelopes short edge first with the flap facing down and to the left. Then slide the left edge guide against the left edge of the envelopes, but not too tightly.

Set the paper thickness lever to the  position.

Set the feeder guard back.

Note:

The thickness of envelopes and their ability to fold vary widely. If the total thickness of the stack of envelopes exceeds 10 mm, press down on the envelopes to flatten them before loading. If print quality declines when a stack of envelopes is loaded, load one envelope at a time. You can load up to 10 envelopes at a time in the sheet feeder. Do not use envelopes that are curled or folded. Flatten the envelope flaps before loading. Flatten the leading edge of the envelope before loading. Avoid using envelopes that are too thin, as they may curl during printing.

For printing instructions, see the following instructions.
See Printing on Envelopes