Disclaimer: Do not do this. If you want a tattoo that bad, go read things to consider before you get a tattoo. Then read Getting a Tattoo. Tattoos will change your life, and not always for the best. I know my tattoos (which are not homemade) have affected mine, whether realize I it or not. This method is dirty, messy, unhygienic, ugly, and just generally bad.

Materials

1. A needle. Basically, any needle will do (sewing needle, safety pin, etc.). The safety pin is probably the best choice, as it's large and easier to handle than a sewing needle.
2. Ink. India ink is usually used, but i've seen pen ink, ink from Sharpies, and lord knows what else. India ink is the best choice.
3. Thread. The thread is used to soak up ink. I suggest white thread so it will be easier to see how much ink it has soaked up.
4. Paper towels. These are used to wipe the skin periodically to clean off excess ink.
5. Something to wet the paper towels. I suggest something anti-bacterial.
6. A willing subject. That means not drunk, high, or messed up in anyway. Nobody wants to wake up with a horrible blotchy bunch of ink under their skin.

Method

1. Draw the design on the skin of the subject. Draw it in pen, but don't worry about being to detailed, as it won't be there for long.
2. Then, take the thread and begin wrapping it around your needle about 2mm away from the point of the needle. Keep wrapping until you have about 8mm of tightly wound thread on the needle. Make sure it's also very thick, as it has to hold a fair amount of ink. You have to somehow secure the thread so it doesn't unwind, which is where using a safety pin is handy. If you're using a safety pin, you can just tie the thread to the circular part of the safety pin.
3. Wet the paper towels with whatever substance your using and keep them in one hand.
4. Dip your needle (with the thread wrapped around it) into your ink. You have to make sure the thread soaks up a good amount of ink.
5. Now, what you want to do is makes points on the subject that outline the design. (Think connect the dots) Place the tip of your needle wherever you want your point, and push down. It doesn't take much, you just want the the tip of your needle to penetrate the skin. The important thing to remember is that you can't be to fast, and you must be consistent.
6. Now that you have the important points done, you can wipe the excess ink off the skin. You will lose most of your outline, but that's why you did the important points first. The skin will be red and raised around the areas that you inked. This is normal, as the skin has just gone through a good amount of trauma.
7. Continue the tattoo, remembering to re-dip the needle into the ink and wipe the skin occasionally.
8. That's about it. Caring for the tattoo is just the same as caring for a shop tattoo. Please read Love and care of your new tattoo.

Things to remember
These tattoos are bad. If your going to get/give one, pick somewhere inconspicuous (I suggest the ankle area). Pick something small and simple. The dots produced by this method are very large, and it's hard to do any detail. Also, please remember it hurts. If you're giving the tattoo, please try to be nice. No digging the needle, no twisting, and push straight down. Also, this is not the same as a kitchen table tattoo or a prison tattoo. Compared to this method, those are very proffesional.