It's probably with the note completely unfolded, but held in the middle of one end so that it's slightly convex. Best done with a crisp one direct from the machine. It is a bit showy though, you can easily end up looking a bit of a cad.

Folded in half is just about OK but anything more than that's gonna really fuck the flow of the person serving you, plus make you look like a walletless hobo.