Using Switches on the Operator Interface

[CronosPrime1]

I know it has been done many times. I know it's often very handy. But how do you install switches and buttons on the OI? I want my robot to dump balls into the corner goals when I push a button or flip a switch. The OI, unlike the RC, does not seem to have convenient digital input connections, only serial ports. How, then, would I connect something like a three-way switch (to control a spike, for instance) to the OI? Please don't tell me to use the buttons on the joysticks. I know how to do that. I'm interested in wiring an actual switch or button to the OI. I know lots of people have done it, but how do you do it?