Step 1: Toss the Racket
Unscrew the flyswatter or Bug Zapper (whatever you prefer to call it). Remove the wires from the racket mesh (what is supposed to electrocute things). You should have two wires, or three for some models, but two of those should be the same color, and you may remove one or twist the ends together.
Step 2: Improvise
What CAN'T you do with high voltage? There are many things that you can do, however I strongly recommend avoiding tazers. I am not responsible for any damage or harm to others (please don't harm others or damage things). What can one do with this power, though? Make a better bug zapper, make a fluorescent flashlight by attaching it to the electrodes of a cfl or other fluorescent light. It doesn't really matter which wire goes to which electrode. This works better if you remove the bulb from the base, which is fairly difficult. If you break the bulb part, put all pieces in a bag and in the trash, using tape to pick up any small pieces. Don't touch the glass! This process should not require a new cfl most of the time, unless the electrodes from inside the glass are lost or the glass is broken. I might explain the process later.
Step 3: Tesla Coil
Attach one of the two wires to your Tesla Coil. Attach the other to one end of a spark gap and the other end of the gap to the Tesla Coil's other end. Ground one side of the coil's wire. Adjust the gap, and turn it on. The gap should arc (DON'T TOUCH IT), and the coil should make small arcs. As always, use caution!
Step 4: Power Supply
Look at the circuit board inside the racket by unscrewing the handle. Clip off the capacitor (that big accoutrement in front that is usually red and rectangular). Do not clip anything else off! If you take that capacitor, as well as others, you can make a capacitor bank. If not, keep it (discharge it in a piece of metal before touching any electrodes or anything) and use your flyswatter circuit as a power supply. The flashlight works better this way. If done correctly, you should have an arc, not a bright flash, as with the capacitor. I have yet to see tutorial show the actual capacitor- less ( not a real word) arc. Have fun and be careful!