The best way to implement the LCD interface is to use a setup as shown in Figure 1 as it is not possible to damage anything when the connection is made in a wrong way e.g. by inserting the cable upside down. On the MCU board a FFC connector with contacts on the top is placed on the top side of the MCU board while on the extension board a connector with contacts on the bottom side is placed on the bottom side of the extension. Recommended connectors are 5-17344592-0 (extension) and 5-1734839-0 (MCU board) from TE Connectivity. The cable itself has the contacts on the same side as indicated in Figure 1.
In order to place the LCD board sideways or mount it on top/bottom it is necessary to use a set of screws and spacers. Board Stacking Options provides the required information to properly stack the board.