When you tattoo, you are driving a needle into the skin. This causes an open wound, and there will probably be bleeding. Make sure your needle is sterile---don't share needles with anyone. You will need to utilize aftercare to help avoid infection. Using a triple antibiotic cream from your local drug store is recommend for the first 48 hours after receiving a tattoo.
The main ingredient is a motor. Get one out of an old cassette player or CD player (and keep the plastic wheel that's on it; you're going to need that). A portable player such as a Walkman is recommended because the motors are small. The motor out of a home stereo unit generally will be big and cumbersome. You also will need wire such as an old extension cord or headphone wires. These work good because there are generally two wires (which you need) already together. You will also need an empty plastic pen like a Bic and one or two rubber bands. A piece of wire will further be necessary for a needle. A guitar string works well because it's thin and stainless steel; paperclips work okay, too. You will also need a frame to put these items onto. The main idea is to have the pen aiming down and the motor on top. Use a toothbrush and bend it into an "L" shape and tape the items to it. Finally, you will need batteries and a piece of paper or cardboard to hold them together. "D" cell batteries generally last longer, but you can use "C", "AA" or even "AAA."
Strip the insulation off of your wires and attach one end to your motor. Wrap paper around the two "D" cell batteries; tape the paper so that the batteries are in a tube like in a flashlight. The other end of the wires are secured to the ends of the batteries. Secure them with tape or a rubber band, but don't do this until you are ready to make the motor go around. Heat it up the toothbrush in the middle and bend it over at a 90-degree angle. Secure your motor near the bend. You may have to adjust it later.
Now sharpen one end of your needle to a fine point. Use emery cloth or 400- to 600-grit fine sand paper. On the other end of the needle, bend about 3/8 inches over at 90 degrees. Clean out your pen. Use a sewing needle to pop the ball out of the end of your ballpoint pen. Cut the pen down to about half and tape it onto the other side of your toothbrush. Stick your needle through the pen and, with about 3/16 inch of the point sticking out of your pen, look at the other end of your needle. Where it is level with your motor, bend it over at a 90-degree angle. This end is heated up and stuck into the plastic wheel on your motor so that it will rotate up and down as the motor goes around. If it is too long, cut it down so that it won't hit the motor as it spins around.
And there you have it. A working tattoo machine. Hook the wires to the batteries and watch it go.