This is my first guide so let me know what ya think
First of all... Make sure you know how you want your mills to look.
Some people spin on the ground more than they turn their bodies. These type of mills generally have straighter legs and look smoother. In this case you need to know how to whip your hips around kinda like when you're doing a flare (Easier than it sounds). Also pushing your body off to the side with your hands will help with speed (speed with respect to the ground as in bird's-eye-view... not rotation of your body).
Other people like to rotate their body around more than they spin on the ground. These mills are generally more elevated and easier to do no-handers with. What I mean by more rotation of the body is that it may take you 2-3 windmills just to get a 360 spin from a bird's eye view... if you spin on the ground more it may only take you 1.5 - 2 mills. In these mills, most people kick their right leg upwards (CCW) when they're linking windmills (facing the ground) and that's what gets them more body rotations.
The Legs
I'm assuming you can do mills now so I don't need to explain the movement of your legs. Just remember to keep them straight and try locking your knees.
To perfect your leg movements however, you need to condition yourself. Most people can do wider splits when they're bent down than if their back is straight. You need to stretch out your front leg muscles located on your thigh by straightening (or even bending backwards) your back as you do your split stretch. If you just can't seem to open you legs up any wider when your back is straight, then close up your legs a little as you're rolling from shoulder to shoulder on your back.
The key is to keep your split constant - it helps your windmills look smoother. Opening and closing your legs will eat up a lot of your momentum. One last thing... bend your heel/ankle, don't point your toes outward. This should be natural when you lock your knees. In my opinion it just looks weird when your toes are pointed outward... your legs look extra long and your feet look extra small.
The Arms: (only if you use them)
As you all know, stabbing windmills aren't the best looking mills around. When you roll off your shoulder to turn onto your hands, pushing off of your right shoulder (CCW) will give you more elevation. This leaves less work to be done by your hands.
Don't use your hands to push off to get more height. Only use them for a little push off to the side so your body keeps a good spin on the ground (if you have to). As you roll onto your hands, your left hand should be placed about 0.5 - 1.0 ft away from your right hand (CCW).
Both your hands should be placed somewhere on the ground right under your neck, not down by your gut like in stabbed mills. Your arms should be in a position like halfway through a push-up with elbows pointing out and backwards. Collapse your left arm by pointing your left elbow toward your right elbow and rolling over it.
The Waist
The bending of your waist is very important to the perfection of your windmills. When you're facing the ground your waist should be almost straight but still bent forward a tiny bit... never bend your back backwards! That loses a ton of momentum and makes your mills look whack.
Now when you're on your back rolling from shoulder to shoulder, make sure your waist doesn't bend forward any more than 90 - 100 degrees or your mills will look ugly. (It will look really flat on the ground and have a big "bouncy" motion to it).
Most people will try this when they're learning since doing the kip up motion helps. Well now you can do them so force yourself to stop. It will make your mills look a lot more circular.
Other Tips
Keep your hips elevated! That way you don't have make your split crooked to avoid hitting the ground. Don't go into a "kung-fu" windmill by kicking into it, start it from other positions such as handspin, standing, or transition from flare.
There are key elements in those methods so try them out and you might learn something. Also try to keep a calm expression on your face when you do your mills, especially if you use your head to do no-handers. Don't make it look like you're hurting by gritting your teeth, frowning or holding your breath etc.