how to mount a flat keypad on a panel
The membrane keypads that I purchase have a layer of adhesive on the back side.

Mark the panel where you want the keypad. Then carefully mark where the flex ribbon comes through the panel. Note that this location is normally inside the outline of the keypad. This allows the keypad to be water-tight because the adhesive seals all the way around the keypad. 

Cut the slot for the flex strip to pass through. Ensure that all sides and corners are deburred and that there are no sharp edges. 

Now simply feed the flex strip through the slot. Peel one edge of the backing paper from the adhesive. Carefully line up that edge where you want the keypad to be and gently stick it down. 

Ensure the keypad is where you want it to be. After you are satisfied with the location, firmly press on that one edge. Now peel the backing paper away from the stuck-down edge and continue to stick the keypad down. 

Ensure that the flex strip is through the slot and finish sticking the keypad down. 

Takes far less time to do this whole procedure than it took to create this reply.