i didnt have a procedure when I did it, I was testing. And Without knowing the specifics of your envelopes, All i can do is give you my experience.
the ONLY time i did this, I was able to complete the project by opening the flaps by hand on all of them. (we later realized we could use a inserter to pull them open, should the job return-it didnt) Otherwise they would open and rip on their own. they were 12x9 envelopes, then opening the flap made them 13 x 9. I layed them face up with the flaps as the trail edge. I also set it to "EIP" mode to slow feeding, because it sat uneven in drawer. this gave it little bit more time to blow the envelope up to the feed head and pull.
i THINK I had 2000 of them to do and took me most of a day to complete.
hopefully i helped you.