A long time ago, I came across a book called The Envelope Mill and purchased it. It came with 3 envelope templates inside it and was packed full of ideas. It is out of print, but you can still find it from time to time, like there's one on eBay at the moment. I have learned to make my own templates using mat board and you can search the Internet for templates.
One of my favorite things to use are old calendars! Last years calendar for me was one we purchased at Chick-fil-a and it's a funny one with cows dressed up as different characters (their slogan is "Eat More Beef")! So, I'll show you how that works up and also show you an old book I have that is pretty useless except for the wonderful photos in it. You can use just about anything you have laying around, I'm fond of using wallpaper!
First thing to do is rip out the parts you want to use. A handheld rotary cutter can be useful with this part.
Next step is to decide which template you want to use, sometimes the size of your paper determines the size of the template.
You simply lay the template down and trace it with a pencil. Cut it out and erase any pencil lines you see.
This is how one from the book looks when cut out. Now, turn it over so you are looking at the back and fold over the two sides (I use a straight edge):
Now fold up the bottom (again using a straight edge) and glue the bottom to the sides, let dry and that's all there is to it!
Here's the finished one:
They are completely legal to use but I recommend you use a white mailing labels for the address and return address.
This page from the calendar would look like something this when finished:
It's fun to see what turns up inside, especially when using calendars!