There are two main options. Using the drum head that comes with pad, you replace the foam inside with piezo sandwich :-) That conversion you can find elsewhere surfing the net. Other posibility, that I will describe here, is using mesh head. Both options offer good results, but mesh heads are much more quieter. 2 Glue it with superglue to form a circle. Resulted circle should be of same size as practice pad's original head rim. 6 Put mesh tube on practice pad, as it was a drumhead, and apply hot glue all...
This is a real heart of the project. Everything is under control of PIC16F84. Microcontroller module. With PIC16F84 running at 10MHz and 8 channel A/D converter MAX155 running at 5MHz. 7805 is missing :-).