There's a standard way to fold an envelope out of an A4 sheet of paper. It looks quite neat and stylish, too, with two alternate corners bevelled. The problem, for some purposes, is that you can't use it to hold small objects, because it's wide open from a big slit at the back. If you were hoping to put a key or some coins in there, it just doesn't work. This is more suited to a letter, where all it has to hold in is ink.
To hold small objects, you can follow the same instructions, but with one extra step in advance. Before following the instructions above, fold the paper in half - just like the first step - but then follow the instructions from the beginning with the doubled paper. You'll get a smaller envelope, but you can open it up, put small items in the middle, then re-fold it. Your items will remain in place.
Mokalus of Borg
PS - I used this just recently for some coins.
PPS - That's also how I discovered the flaw in the original design.