Prepare your colours* by adding water. The more water the paler the result.
I am using three, an oriental red, a buttercup yellow, and a mixture of the two with some added pearl.
Spritz the fabric lightly with water, and prepare a sea sponge (this gives a better effect than man made).
Spray the sponge with lightly with water before beginning. (This helps the paint to be more evenly distributed)
Dip into paint, then remove some of the excess paint onto a spare piece of fabric before using it on main fabric as shown below. If you don't do this you end up getting big splashes of paint instead of the delicate marks we are after.
With a VERY gentle touch lightly dab the sponge across the fabric. Move the sponge around so that you don't get a repeating pattern forming from the shapes on the sponge.
Different effects are obtained by varying the amount of paint and water (and dabbing) used.
Above has little paint and lots of white showing through.
Below is quite colour intensive.
This one is somewhere in between.
This method of painting fabric dries quickly and uses little paint. When I demonstrate this in class I find that the most common mistake is to be too heavy handed, but with a little practice it is easy to do.
*I use Pebeo Setacolor textile paints