Printing on envelopes for invitations - how did you do it?

I am trying but I don't think it's going to work - I cut out a bunch of pieces of paper as templates to practice but I can't seem to get everything centered correctly. How did you do it?
(repost from bhb)

I'm getting this done with the printers of the invitations; but I printed diy rsvp envelopes and they came out fine.

You need to set the printer to a custom size.I manually fed the envelopes into the printer vertically, they got stuck when I fed them in horizontally.