When your muscles are warm and relaxed! You should stretch after you have done a general body warm up of about 5-10 minutes. Marching or jogging on the spot in order to get the heart rate and body temperature up can do the trick.
Dynamic stretches are used prior to walking and they are essential in preparing your muscles for the quick movements that occur with walking. Static stretches, on the other hand, are more useful to improve your overall flexibility and are most effectively done after you have finished your workout.