Thanks very much, it looks like your link is going to be the solution I need! It's very helpful.
But I would like to ask one clarification;
In your instructions, you say to staple the envelope I need on a letter sized paper.
Is that just to work out the margins, or will I need to do that to print all my envelopes? I mean, will I have to print all my envelopes stapled to letter sized paper?

I see that your instructions specify setting up the page as Portrait, not landscape. My printer seems to be set up so that the normal way to print envelopes is to insert the envelope's front left side, face down into the printer. So that would mean it would normally need to print landscape.
But if I have to print this portrait, that seems to suggest I do have to staple the envelope to the paper to print it, because I don't think the envelope is tall enough for the printer to grab it if it isn't stapled to letter paper.
Do I have this right?