Windmills challenge you to keep your body stacked and stable while you’re changing positions. The motion here is a simple hinge from the hips, but you need to keep your overhead arm stacked directly over your shoulder. Practice with light weights at first, and move up when you’re ready. You may also want to try holding a second kettlebell in your bottom hand, so that you’re deadlifting it each time you stand up.
This video includes some tips on how to hold a kettlebell overhead, in case you didn’t quite figure that out when you were doing carries. Instead of holding it in the middle of the handle, you’ll want to find a comfortable position with your hand near the corner of the handle and the body of the bell resting on, but not crushing, your forearm.