It's called a Z fold and is not my design. I can't remember where I learned it from. I wrote down the directions and still have to follow them.
Start with a piece of paper 51/2 x 12. Score at 2",4",8",10".
Go in at 11/2 from the top cut from 2" to 10"
Go in at 4" from the top cut from 2" to 10"
Fan fold up down up down
Take the middle score line and push in opposite directions.
I've attached pictures at different levels of opening. I hope this helps!
When folded, it fits into a standard A2 envelope