Canon imagePRESS C800 Series Troubleshooting Manual page 106

Chapter 1
4) Place the envelopes on a clean, level surface, and press all the way around the envelopes by hand, in the direction of the arrows, to remove any curls. Repeat this
step five times for each set of five envelopes.
5) Hold down the four corners of the envelopes firmly, so that they and the sealed or glued portion stay flat.
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