You can check out howaboutorange.blogspot.com's original tutorial by clicking here.
1) Find an envelope of any size and seal it shut. Embellish the envelope, if you want. These can be kid-cute or sophisticated, just by adjusting how you choose to decorate them. You can also print a design or text on the envelope by running it through your printer.
2) Cut off one end of the envelope (Decorative scissors make a nice finish).
3) Fold one side in toward the center (maybe about an inch or slightly less). Do the same with the other side, then fold the bottom up by the same amount, making creases in the paper.
4) Put the bag over one hand. Use your other hand to shape a flat bottom for your bag, pushing the bottom center crease downward, causing the side folds to buckle out. Crease the corners of the bottom into triangles.5) Use a bit of double-sided tape to stick the corner triangle flaps to the bottom of the bag. Reverse the folds along the sides of the bag to give it shape.
8) If you like, punch holes and thread ribbon through to create handles or to tie the bag closed.