The original kickdowns are mounted as shown in this picture:

Notice the curved tang that's bolted to the throttle shaft. This is what engages the kickdown around 3/4 throttle.
If you have a non-stock carburetor, you can buy B&M or Lokar kickdown.

Here is a shot with a Lokar setup installed:

Basically a wire pull system that at 3/4 throttle, pulls a wire tripping the switch and sending the signal to the transmission.