Start by bending at the waist until your fingers touch the ground, keeping your legs straight.
Walk your hands slowly away from your feet, while keeping your legs straight. As your hands get further away from your feet, brace your core.
Walk your hands out as far as you can while keeping a flat back and your legs straight. When you reach your limit, start to "tip-toe" your feet toward your stationary hands. Walk your feet as close to your hands as they will go, while keeping your legs straight. That's one rep.
Repeat the process of walking your hands out and your feet in for given amount of reps.