Start with the easier exercises from this video and then progress to lifting your knees up or extending your leg.
Posture and position are important as well as stabilizing the trunk. Make sure your hips and sides are not swaying with arm or leg reaching.
(Progression of exercises)
Shifting your weight is a great place to start to get some rotator cuff stability.
Arm reaching with one hand straight, thumb up.
Lifting a knee while stabilizing at the shoulders.
Weight bearing with reaching to the ceiling is called the 'John Travolta' and is a great stabilizing exercise.
Single leg extensions.
Both knees off the ground. This will make you work.