You can make a paper envelope from wide ruled paper using only the paper itself. You can write a letter or message on the inside of the sheet of paper that becomes the envelope, or you can place another sheet inside the envelope you create.
To make your envelope, fold both top corners down to where they meet the outside edge of the paper, about three quarters of the way down the page, and then unfold them, leaving two creases. Next, fold the two creases in on themselves. This is done by folding each crease in from the vertical center on the outside of the paper to the vertical center in the middle of the paper. Then, fold the ends of the triangle that you just made to the top of the triangle. Next, fold the tops of these little triangles down so they form two creased squares. Take the ends of both of these squares, and fold them down to the point of the lower, downward-facing triangle. This is the bottom of your first folds. At this point, you have a small triangle between two larger triangles. Take this smaller triangle and fold it up, pushing its top point under the flap that you have created. Next, fold in the two square sides at the bottom to where they meet in the middle, and then fold the newly created corners in to form another triangle. Now you should have a triangle at the bottom and a flap at the top. Insert the triangle at the bottom into the flap at the top, and your envelope is complete.