Start from an Idle. After a left handstick hit, let the devilstick start rolling over the right handstick, as if you were going to do a propeller. As the devilstick is rolling over the right handstick, cross your left hand over your right. When the devilstick falls off the right handstick you should have your left handstick over your right arm to do a left handed backhand hit. This will start the devilstick spinning in the opposite direction. Roll it over your right handstick again, the opposite way. Uncross your left arm and let the devilstick roll over the left handstick. As it is rolling over the left handstick cross your right arm over your left. When the devilstick has rolled over the left handstick do a backhand hit with the right handstick. Roll it back over the left handstick and at this point you have done one cycle of mills mess for the devilstick.
Here is a picture of me doing mills mess.