Break off the tabs on the sides of the piezo transducer's plastic case, then crush the case around the edges with a pair of pliers. Work slowly and steadily so as not to break the transducer inside. Avoid crushing the area where the wires exit the case, as they can be damaged relatively easily. Pry apart the case and carefully remove the piezo transducer and attached wires.
Cut the audio cable to the desired length. At the severed end, carefully remove about an inch of the outer rubber with the razor. The easiest way to do this is to make a shallow lengthwise incision at the edge, peel the rubber back, then cut off the excess. Gather the outer wires and twist them so that they stay together.
Remove a half inch of the inner rubber so that inner rubber and wires are protruding from the cable by half an inch. Gather the inner wires and twist them so that they stay together.
Attach the piezo transducer's red wire to the outer wire of the cable, and the black wire to the inner wire of the cable. There are several ways to do this, although soldering is the most effective and secure. Other options include twisting the wires together, then hot-gluing them or taping them with electrical tape.
Wrap electrical tape tightly around the area where the wires come together, even if you have taped the wires together. This will make the connection look neater, and also offer increased protection.
Attach the piezo transducer to the surface to be miked, then plug the other end of the audio cable into a preamplifier to amplify or record the sound. Though a contact microphone can pick up vibrations from any surface, smooth and relatively flat surfaces are ideal. For example, it can be held against the throat while speaking or singing, or taped to the body of an acoustic guitar.