Flat Keypad or Membrane Keypads are cheap and beautiful Keyboad that can be used in systems. but the question is how can they be mounted in a box or panel?
Figures 1, 2 and 3. Cut the slot, mount the keypad and route the ribbon cable. Image source larger view and further instructions from liudr.
The ribbon cable is attached to the keypad some distance back from the near edge of your photo. The datasheet (and if it doesn't have one then don't buy it) should show the slot dimensions to turn the ribbon cable through the panel without excessive bend radius. The panel can then be glued to the front panel - usually with the built-in pressure sensitive adhesive.
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.