Place the stack of mail on the feed deck. Place the envelopes with the flap side facing down and the top edge against the rear wall.
Slide the side guide up to the mail stack. Leave about 1/8" space (about the thickness of 2 quarters) between the stack and the side guide to ensure the stack feeds without resistance.
Slide the stack into the feeder until you feel resistance. The image shows how far into the feeder you need to position the stack. Note the side guide is not in the correct position here.
UPDATED: January 15, 2021