Do not "wash" batting. You will end up with the world's largest cotton ball and it will be totally useless for batting.
If you wish to _pre-shrink_ batting put it in the washer and fill with hot water. Turn the washer off and let it sit for 15 minutes. Turn the dial to spin and turn the washer back on. After the water has spun out put the batting in the dryer on high. Check it after 10 minutes or so. You will probably have to unwind it and put the damp sides to the outside.