I know it. Used to know half the protocol by heart.
What you need to do is figure out a way to convert the physical action to some sort of electronical signal. This means, some sort of circuit that detects whenever the kick is hit, and some sort of signal that is in relation to the velocity of the hit.
From there on the circuitry is easy and straight forward. The midi protocol is an easy historical protocol that can be handled by the most simple microcontrollers. You program it to receive the hit-signal, measure the velocity, and then send out the note on message. And the note off message right afterwards probably. (I don't really know the details about how note off is handled by these kind of controllers.) It's really easy to program that.
The hard part is the velocity really. I haven't got a clue how that is done. I know you can easily make this kindof pad by putting one of those piezo-speakers (that also function as a mic) under some rubber. Don't know how they handle velocity, could be that you can just measure the output of the mic. (Easily done by the builton ADC's of a microcontroller.)
The hardest part in making these pads, as some drummer once told me, is finding the right amount of rubber etc, to get a good physical response of the 'drum'....