The all-cotton batting I use is "Warm and Natural." Instructions for pre-shrinking are included when I buy it.
That said, I disobey those instructions most of the time. Because I have a front-loading washer (which is gentle to begin with), I wash the batting in it using the hand-wash setting with warm water. Then I dry it in my dryer on the delicate setting. Seems to work just fine.
Good luck!