First, I made the cushion cover from unbleached calico - how I did that is another subject! Before painting on any fabric, first wash it to remove any coating that may have been put on it during production. This will help with the surface you are painting on, it removes any 'waxiness' that might prevent the paint sinking in to the fabric.
Here I have painted on calico, but I have previously painted on cotton, t-shirts and silk - each fabric is different in absorbency and it is wise to have a trial piece before you go in for your main project. Calico is robust and like painting on canvas; t-shirts are slightly more absorbant and need drier paint mixes and silk is extremely absorbant and needs patience and caution!
Cover the top of your painting with muslin or another teatowel, just to prevent any mishaps. Occasionally my iron decides to spew out some brown water due to limescale - you do not want this spoiling all your hard work!
Have fun - it's great to have your work of art on something different for a change!