how to mill... (bare with me it is hard to explain. pictures and videos make it easier.)
the main part of mills is the placement of your arms and your legs.
for starters windmills, you should stab. stabbing is when you have your elbow on one hand 'stabbed' into your stomach like the baby pose. this will allow space for your legs to pass through before you roll over to your back. the other hand will block your face and help push. for more advanced people, you can either mill with your hands tight to your chest, or in front of you like you are going to block a kick.
your legs should be in a wide v shape the whole time. the wider your legs are, the better. stay on your upper back as much as you can, even get close to your shoulders.
look up videos online, or e-mail me and i can send you some vids.