Train your body to be physically fit and have a lot of upper body strength. In order to do the windmill, you must have high levels of ability and fitness. Find yourself a comfortable environment with a lot of space and, if possible, a soft floor to avoid injuring yourself.
Stretch your body out. Make sure you are fully warmed up before attempting to learn how to windmill.
Place yourself in the "hand glide" position. Touch your stomach with your left elbow and place your weight onto your left hand. Lie down with your face facing the floor. Your body should form a tripod shape so you can support yourself. Practice maneuvering yourself into this position. Try taking your right hand off the floor and balancing your body with just your left palm. Remember to rest whenever your body feels like it.
Attempt the "fall down" position. From the "hand glide" position, push your body forward with your left hand so your body slides to the floor slowly. This requires a lot of body strength, so keep practicing if it doesn't work on the first try. Your left hand should be close to your head, your body flat and your legs widespread in the air. Practice this movement until it flows naturally before continuing.
Return to your first "hand glide" position. Now we are going to complete a full rotation. After falling into the "collapse," flip your body around and kick your legs into the air. At the same time, you must push your weight onto your upper back and left hand and then transfer the weight onto your right hand. In order to achieve this, you must have a good flow and swinging motion. All three movements need to happen simultaneously to create the first rotation of the windmill.
Keep practicing these steps until you have completed a perfect single rotation of a windmill. This will take some time, so be patient and work your body carefully. Once you have mastered this step, you can continue practicing these moves. Eventually the momentum will flow into various rotations and you will be windmilling all over the floor.