Before you try and build this DIY taser, beware— you could go to jail if you go around tasing people with it. Just having one could give you a fine, because they're not legal in some cities and states. If you still want to make one, proceed ahead.
In order to build this DIY electric shock taser, you'll need to have some knowledge of electronics and circuitry. You can get the schematics and board transfer at Mechanical Mashup.
This taser is actually more like a cattle prod, which gives a good jolt of pain. It's jam-packed with about 1,200 volts. That's more voltage than one of those barking dog collars and more than an electric fence. But a real taser runs at about 10,000 volts, which is what you really need to really effect the central nervous system.