Start Position:
Place hands under shoulders and knees under hips, maintaining a neutral spine. Keep you head on top of your shoulders and feel width across the collar bone to soften the rib cage. You should also feel your abdominals muscles feeling slightly drawing towards the middle to support your body on movement.
Modification: use stability balls under your knees or hands or use toning balls under your hands to challenge your stability further.
Keeping the spine in neutral slowing draw the leg along the floor and lift no higher than hip height. You should feel your gluts (buttocks) engage to support the leg in the air. The body should maintain the same shape as the start position.
Modification: If lifting the leg brings you off balance you could just draw the foot along the ground as below. Feel like you are pressing a weight away from you to get the buttocks working.
Come back to the start position each time and alternate legs or arms.
To increase the challenge you can raise both arm and opposite leg and swap over.