I also wanted it to fit in a standard envelope. After some googling and pinteresting, I found this tutorial by CARDZTV.
Don't let it intimidate you. It will take some time for the first one. OK it took me a hour lol but then I couldn't stop when I finished the first one! I ended up making 8 in total.
I used my Hello Kitty paper pack and scraps! Hope you try it too!