The Smash:

•  Get into position behind and beneath the shuttle.
•  Point your non-racquet shoulder toward your opponent and shift your weight onto your back (racquet) foot. 
• Hold both arms up. Hold your non-racquet arm out in front of you for balance, and your racquet arm up behind you with your elbow bent 90 degrees and your wrist cocked. The racquet head should be behind
  your shoulders. ( Image a.)
•  Begin your swing from your shoulder while opening your torso toward your opponent. 
• Reach up for the shuttle, jumping off your back leg and transferring your weight to your forward foot. 
• Strike the shuttle as high as you can reach it, while it is still out in
  front you. ( Image b.) 
• Snap your wrist, as you contact the shuttle at least three or four feet above the net level and in front of your body. 
• Hit the shuttle with a closed racquet face. ( Image c.)
•  Follow through, keeping your arm high, while your wrist snaps completely through the shuttle. Then swing your arm down and through the shot
  

Source : ehow.com