First of all, eat your cereal bars. You'll be left with an empty box, like so:
Next up, cut along one of the vertical folds, like so:
If you turn the loose panel over, you can mark out where you need to trim each edge. Take about a centimetre of each side, otherwise the finished envelope will be too bulky to tape together. You also need to trim a little bit off one of the top edge, but I forgot to take a photo of that part.
Here's the same thing in inches, for my international/older readers: