If you have a score board (there are several brands; here I used the Martha Stewart version) and a paper trimmer it couldn't be easier, but you can certainly use a stylus, ruler, and cutting mat to score your lines, and trim the papers by hand.
For envelopes that work with standard A2 cards (folded size 5 1/2 x 4 1/4") first cut your calendar pages to 8 3/8" squares.
Place your trimmed page, picture side down, on the scoring board like this:
You just have to make sure the points touch the center marks (6") before you start scoring. This is what I mean:
You now score at the 3" mark on your board. Then rotate the page so you can score the opposite side also at 3". Next, score the two remaining sides at 3 5/8". Your scoring will look like this (I darkened the score lines with a marker so you can see them):
Fold over the sides you scored at 3" first, and use a bone folder to get a good crisp fold.
Apply a line of adhesive on the lower edge of each flap and fold the bottom flap up. I used my ATG, but a good quality glue stick will also work.
Your envelope is ready to receive whatever you're mailing.
All you need to do is apply two address labels so the P.O. isn't distracted by all the colors, LOL.
The first batch I made were from the 2012 Cloth Paper Scissors calendar, so they are especially beautiful, though my crappy photos don't do them justice.
But you can use any paper that is sturdy enough. And if you can't get an 8 3/8" square out of your paper, you can always make a smaller envelope. This one is just the right size for sending out an ATC, and it was made from the cover of a Dick Blick catalog. (Note: if you're making a smaller one, you will have to adjust where to line up your points on the scoring board. For this envelope, they lined up at the 5 7/8" mark instead of 6").
I had lots of colorful strips left over after trimming my calendar pages, so I decided not to let them go to waste. I wove them to create a substrate for some journal pages. This is what the back looks like (because I forgot to take a photo of the front before I started throwing paint around):
And after applying collage elements, paint, stencils, inks, more paint, more stencils, more inks, some sprays, and even rub-ons, it is pretty much unrecognizable. And still a work in progress. But I plan to cut it in half and make it part of my 2013 Sketchbook Project....but that is a post for another day.