Lets see how to put it together:
Step 1: Material List and Schematic
As you can see the materials needed are:
1 On/Off switch, (preferably a round key switch witch can fit in the handlebar end of your bike.
1 mercury switch
1 2.5K resistor
1 10K resistor
1 4.7μF capacitor
1 BD139 transistor, (although any NPN transistor will fit)
Also - (not shown here), 1 12V battery, (the ones used in car alarm transmitters)
Step 2: The Final Assembly
Step 3: Mounting
Good luck and enjoy.