1. Kneel with knees hip-width apart and hands firmly on a bench about shoulder-width apart. Brace the abdominals.
2. Practice lifting one hand and the opposite knee just an inch or two off the floor while balancing on the other hand and knee and keeping your weight centered.
3. When you feel steady and ready to move on to full range of motion, point the arm out straight in front and extend the opposite leg behind you. You should form one straight line from your hand to your foot, keeping hips squared to the bench. If your low back begins to sag, raise your leg only as high as you can while keeping your back straight.
4. Hold for a few seconds, then return your hands and knees.
5. Switch to the other side.
6. Keep the abs engaged throughout the entire exercise, and work to minimize any extra motion in your hips during the weight shift.