The first step is to decide what picture you would like on your shirt. Once you have an idea, lightly draw it on with a pencil.

Now that the design is on the shirt, it is time to prepare the paint. Take your textile medium and your paint and mix it together according to the instructions. For mine, I had to mix the same amount of textile medium and paint. You can also use fabric paint if that's what you have. :)
After that, put a piece of cardboard in between the front and back of the shirt, this will prevent the paint from bleeding on to the other side. And now, you can paint!

Let your painting dry, and then iron it according to the instructions. Now you are ready to rock your newly styled shirt!