This PCB will serve as a breakout board to connect the main control board to several sensors. Since it is a flex PCB the male connectors will be a part of the PCB and not soldered connectors and I’m a little bit lost when it comes to selecting the right layers.
I would create a footprint with smd pads on top and bottom.
To ensure that there is no soldermask between the pads i would fill the area between this pads with graphical lines on the appropriate mask layers.
I played around a bit and made a connector to show you how something like that could look like.
I assume you want an endresult looking somwhat like that.
Edit:
if you have the pins on both sides or just one depends on your specific usecase.
If you use both sides you could also have different pins on top and bottom and double your connector pins. Again depends on your application.
(Or in other words: it depends on the female side of your connector.)