Can I make a suggestion, as a (completely amateur) calligrapher who also did her own envelopes?
For your own sanity, address an envelope to yourself and make it perfect. Put a dummy card–like a card you don’t care about just so it’ll have the weight and stiffness of normal mail. Put the stamp on. Now, send it through the mail.
When you get it back, observe how it looks. This is how it’s going to look when your guests receive it. And that’s BEFORE they glance at the front for .02 seconds to ascertain it is, in fact, for them, rip open the back, and toss the remnants on the floor of the birdcage for their canary to poop on.
And I say this as someone who spent a LOT of time and $$ on her invites because I’m that kind of person.
Okay, more advice along the lines you’re thinking:
1. Buy at least 20% overage of your envelopes. Some people can get by with 10% more, but I like at least 20 for peace of mind.
2. I don’t see the need for pencil before pen–it’s a lot of work and you’ll see the ghost of pencil underneath and you have to erase it and blah blah blah. Just use pen.
3. Cascading is easier than centering. Cascading is starting from the left, then indenting every line towards the right. It still looks special and formal and is much more forgiving.
4. Consider making a guidesheet that can be slipped into your envelope (if unlined) that just has the lines so you can follow the lines and make yours straight.
5. Use a pen that you feel confident using. Sounds weird, but there are pens that I like because their drag is good and the ink is clean and blah blah blah and pens that for some reason, the physics just aren’t working so well. Do a test run with different pens and one envelope (just write on it a bunch of times) and see how the pen reacts to the fibers. Some pens will do fine; others might feather; others might bleed through. You won’t know until you try it.
And really, most pens that bleed through are also going to feather, so if the pen requires you to insert a blotter, I’d just use a different pen that doesn’t go through if you can.