Place your hands slightly wider than shoulders; keep torso straight and head aligned with spine throughout the exercise. Lower yourself until the chest touches the floor; press back to starting position.

Alternatives (in order of increasing difficulty)

Counter-top: place hands on edge of counter, move feet away until body is at 45 degree angle to the counter; perform as above.
Bent knee: kneel on floor and bend knees so that you pivot off the knees rather than the feet.
Regular: described above
Feet on bench: place feet on sturdy bench about 2 ft. tall. Perform as above. Make sure body stays straight!