In a wider than shoulder stance drive your hips back and to the side while extending the opposite leg. The loaded leg should function as though you are performing a normal squat. First your hips travel back then your knees track over your feet. Keep your chest upright using the dumbbell as a counterbalance only squatting as deep as you can while maintaining neutral spine.
Just like an air squat your knees should track over your feet, but not past your toes. Likewise your heels should remain on the ground. Cossack Squats can Also be done while holding on to a railing or similarly sturdy object.