Warning: if you screw up with this you could electricute yourself.

Step 1: Materials

For this you will need;

4 or 5 Disposable flash cameras (I went to a camera store and asked them for some and they took a big box with about a hundred in it, they sold them to me at ten cents a camera, I only got ten)
aluminum foil
plexiglass
wire
small switch
big switch
aluminum or steel rails
6 nuts and bolts
wire hanger

tools;

screwdriver
soldering iron
duct tape
wire strippers
wire cutters

Step 2: Prepare the Cameras

break open the casing on 4 or 5 cameras
take out the circuit board
take out the capacitors from all but one of them (you might want to take out the flashbulbs too they look cool when you put several thousand volts through them)
remove the swich thing and solder two wires in its place

Step 3: The Circuit

solder the small switch to the two wires you made in the last step
solder the extra capacitors onto the current on (remember to keep the positive and negative sides together)
solder two long wires to the capacitors
solder the big switch onto the positive wire

Step 4: The Rails

make two aluminum or steel rails about 1/2in by 5in (make sure one long side of each rail is perfectly straight)
cut two pices of plexiglass about 1.5in by 4.5in
make a small gap between the rails that is perfectly straight (I used a wire hanger as a divider)
sandwitch it beween the two pieces of plexiglass and clamp it
drill six holes for the bolts
put the bolts in and screw the nuts on tight

Step 5: Set It Up

clip the positive wire on to the right rail and the negative wire on to the left rail with alligator clips
ball up a small piece of aluminum foil and jam it into the middle of the rails
put a battery into the battery thing

Step 6: Fire!

press the small button to charge up the capacitor bank
when the led blinks you can press the big button and fire it
after you're done use a srewdriver to trigger the capacitors a few times before touching it