So without further ado, we shall begin.
I'm going to start with the most basic way.
Write whatever you choose on one side of a blank piece of the paper of your choice. I recommend using the inkiest (preferably black) pen you have to get the closest to a dipping-ink effect.
Turn the paper sideways, and fold down one side, as shown below.
Fold the other end in a similar fashion.
Fold the bottom up.
Fold the top down. Are you getting bored yet? ;)
Now you can see that it sort of makes an envelope by itself-- all you do is tuck the bottom bit into the top flap.
You could leave it that way, but if you have anything with which to seal it, that would be ideal. You can get sealing wax online... perhaps at some craft stores or something. I've tried normal candle wax, and that doesn't work too well. I have messy experiences with that, haha. Like I was talking about in my JA party post, I used to use regular glue-- sort of make a round blob with it and let it dry overnight, and then paint it and pretend it's wax. That can be a little tacky, and is prone to tear the envelope. Haha.
Here's a picture of a Regency-style letter I got from a friend, and she used sealing wax, along with some sort of seal. (Or perhaps button-- I have one I like to use.) The seal is broken, obviously.
Write the name of the addressee on the front.
If you're sending this in the mail, you'll either need to fold it to make sure it meets post office size regulations (at least 3.5 x 5 inches... talking U.S. here, of course) or what might be easier is just to put it in another envelope.
So that's that! As you all know, I've watched a looot of period dramas. And in so doing I've observed a lot of old-fashioned letters, and it sometimes amuses me to try to copy the way they're folded... so here are a couple more ways to fold old-fashioned letters that I've picked up.
Start with the paper sideways like this...
Then fold the bottom up and the top down, like this.
Then fold in two flaps...
And it will look something like this.
Now onto the third example...
Start with something like this, and then fold once, and again, where indicated.
Then you tuck it in just like the first example.
So there you are, anyone who was interested. :)