How do you put a wallet in your back pocket correctly flat side up or the side that opens?

Short Answer: Flat part/side (or what I would call the *fold part/side*) up.

Long Answer: Flat side and/or *fold side* up for a wallet that folds at a single point in the center of the material. This type of wallet is called a "bi-fold" and is probably the most common representation of a wallet.

My preference is the "tri-fold", which folds at two points, sectioning the wallet into thirds. The only way you could put this in your pocket "incorrectly" is by trying to squeeze it in horizontally or I guess if your pants have extra large pockets...XD