Each button is made of 3 parts - theres the plastic part that you press, beneath that is the elastomer which is a rubber molded piece with a conductive bit that goes underneath the plastic part, and finally on the PCB there are two interdigitated and exposed traces. When the user presses the plastic button, it pushes down on the elastomer which then pushes the conductive rubber onto both traces, shorting them.
There is also a black blob in the middle. This blob is a chip that takes all the button inputs and then converts it into the way that the SNES wants to hear. Thats all fine, but we dont want to use the blob because we are going to make our own custom chip software. (Note that it would be pretty easy to make the Teensy 'talk' right to the blob using the SNES protocol but then you wouldn't be able to adapt this tutorial to other controllers, for that reason we're going to do it the 'hard way')
The question is now how can we listen to all the buttons?
Note that this is really just a symbolic schematic, the ground wire doesn't necessarily connect on the side thats indicated, we're just showing how all the buttons have a common ground pin!
OK now this is straight forward, if you are not sure how to read buttons with a microcontroller, we have a nice tutorial you might want to check out (in fact, we really suggest it as we'll be referring to concepts in that tutorial) Basically each button connects to an input of the microcontroller. We'll need a pull-up resistor, but luckily we can set the microcontroller's internal pullups (so we dont have to solder in 12 10K resistors!) Then the microcontroller can listen on each pin for a button press and when it is received, generate a keypress event.