This is a super simple card I came up with when I was looking at some similar stationary someone gave me.
Also, the lamp was on on my desk so I didn't have very good lighting to work with, sorry my pics are so dark!!
You can use any rectangular piece of paper for this. For the How-To I just grabbed an 8.5x11 sheet of computer paper. It should also be noted that this is kinda hard to do with card stock, possible, but hard. I stick with thin sheets of paper.
First, looking at the paper long-ways, you measure and mark one inch from the bottom left corner and top right. Use small pencil marks so you can erase later, I'll use a marker to really show you where the marks are.
Then you measure and mark three inches over from the bottom right corner and then top left corner.
You may have to enlarge this picture, but this is how it should look. (Minus the large marker marks, of course.)
Then, take the bottom right corner and fold it in so that you are folding at the 1 inch mark at the top and the 3 inch mark at the bottom. Do the same for the top right corner too!
Then you will fold the bottom up a little less than 1/3 so that the sides line up together, It will look like a slant. Then fold the top down over everything matching those sides too.
When you open it up, it should have all these creases.
That's it! You can make it pretty and you can add a card in the middle square (just make it the same size as the middle rectangle and fold it up in the envelope) or you can write your letter right on the inside, saves paper and time! You can make any size and out of any paper!! woo!