How to tye-die T-shirts

Step 1: Things You'll Need

Urea
Dyes
Cotton shirts (silk works too, polyester is the problem fiber)
Rubber bands or other tying implements

So, tye-dying is really simple. All you need are some dies, a fixer, some fabric, and something to do the tying with.
I like the Procion Dyes--They're very bright and are easy to mix. You can get them online, or at your local art store. Get a bunch of colors
You'll also want urea. You can get a bag of this at your local art store--it usually comes as a bag of these small white balls
Get some kind of fabric to die--100% cotton works best, bust silk works too, and denim does, to some extent. Polyester, rayon, and the like won't work at all.
Finally, get some rubber bands or twine.

Step 2: Mix the Dyes

So, the tye-dye basics are like this:
you have some dyes that are water-soluble. If you just squirt them on a shirt, they'll wash off. However, if you treat the shirt with a fixer, it'll hold the dyes in place. In our case, urea is our fixer.
I really like mixing my dyes in ketchup-style squirt bottles, which makes for easy dying. However you choose to do it, add some warm water to a container, and then mix some dye powder in. I do it qualitatively--the more dye you add, the brighter the colors will be, although there is some saturation point. A good rule of thumb might be 1 tablespoon of dye for 8 ounces of water.
Anyway, put some dye into the water and mix it up. Then, go on to slep 3

Step 3: Soak the Shirts

So now we're going to soak your fabrics in urea to add fixer.
First, get some tub that's large enough to hold your fabrics and water. A sawed-off 5 gallon bottle worked well for me.
Now, add enough warm water to allow you to submerge your shirts.
Now, add the urea. Again, this is kind of qualitative, but the stronger your urea concentration, the better your dye will stick. For a few t-shirts, usually a couple handfuls of urea is fine--try to put in as much as you can dissolve.
Now, stick your shirts into this, get them nice and soaked, and let them stew for five minutes or so

Step 4: The Tying Part

So now, we'll tie the shirts, which determines what pattern comes out.
First, take the shirt out of the urea and wring it out. A wringer/washer is gratuitous, but makes this part endlessly more fun.
Next, twist/pleat/twirl your shirt into whatever pattern you like. It's impossible screw up tie dye, so just do whatever interests you here. Some basic batterns for swirls and lines are shown below.
Now, add rubber bands anywhere you want a different color. You'll want to be sure that they're on tightly, so double them over a few times
Now, you're ready to dye!

Step 5: The Dyeing Shtick

So now you're all tied. Sweet. Let's get to the fun bit.
First of all, you should know that these dyes will stain your skin for a few days. If that bothers you, wear gloves.
Also, do this somewhere that you can clean easily or you don't mind being colored(i.e. a sink, or a metal work table)
Now, take your dyes and put a color into each region that you've banded off. Be sure to get dye into all the folds, otherwise you'll have a lot of white.
Now, flip the shirt over and put the same color dyes on the other side.
When you're done and satisfied, put the shirt in a plastic grocery bag so it doesn't dry out, and let it sit overnight.

Step 6: Results!

After a sleepless eager night, rush to your tie dying area and take your shirt out of the bag. It's still in the danger-will-stain-your-fingers mode.
Take the rubber bands off and unfurl your creation. Cool, huh?
Now, rinse the shirt under color water, squeezing and wringing it, until color stops washing out of it.
Now, throw it in the wash. For the first wash, it still might bleed a bit, so throw it in with other tie dye or by itself. Generally, it's pretty benign after you've rinsed it out.

Hooray! you've dyed a tie!