I did it only once with a cross stitch pattern . It was a Christmas tree skirt that I didn't want "bright" white . I just filled the sink with hot water and dropped some tea bags in with the fabric, let it sit until it was as dark as I wanted , pulled it out and let it dry.
I am sure there are more complete directions on the net.
I think if I was doing it for a quilt I would do the fabrics seperately , that way you can control how much each fabric changes . But that is just me