You might put it in your washer and let the washer fill with the type of water the quilt will be washed in. Let it soak at least until the water is cool. Then spin the water out of the washer. Take care though, don't let the washer agitate or you won't have any batting left! (Hm m m been there, done that! LOL)
Next put the batting in the dryer to dry until just damp. Let the batting air dry the rest of the way.
You also might want to check the size of the batting before and after you do this procedure. You may find that the brand you have really doesn't shrink and you can save yourself some time in the future,