Textile medium will hold it well. You just paint it on with a brush.
Depending on how saturated you want the color, you can either work with the colors wet (more saturated) or dry (more transparent). When blending, I will paint on the medium immediately. Otherwise, you can wait until the fabric feels dry to the touch and then I use a 2" brush to paint medium over the whole design. I hang mine to dry for the recommended time. It will get a sheen on it & feel a little stiff until washed.
It does get slightly lighter the first time it's washed. If you want a particular tone of a color, I highly recommend experimenting first on scraps.
Have fun! Can't wait to see the results!!!