I have used acrylics on fabric in a number of ways: you can mix it with a textile medium, paint, let it dry and then heat set with an iron. You can put some acrylic in water and soak the fabric (kind of like dying, but using the acrylic paints), and this will wash out to a lighter color depending on how much paint you put in the water. If you just water down the acrylic paint and paint on as usual, without the textile medium, even with heat setting, you will likely lose quite a bit of color. A cool alternative now is to use colored pencils, best ones are by Derwent, called Inktense, color with them, like on paper, and then paint over with a textile medium. Colors become really intense, and permanent.
If going for something like a tiedye, you'd want to use the sticks as opposed to the pencils though.