HP 3520 - Manual (Page 23)

Remove all paper from the main input tray.
c. Load envelopes.
Insert one or more envelopes in center of the input tray. The side to be printed on should face down.
The flap should be on the left side and facing up.
Slide stack of envelopes down until it stops.
Slide paper-width guide to the right against stack of envelopes until it stops.
Load media 21