Proper Technique:
For wrist flexors, stand with the arm extended in front, fingers facing outward and pull fingers/knuckles towards you with the other hand
For the wrist extensors, extend the arm so that fingers face inward and pull the fingers and hand towards you with the other hand
Targeted Muscles
Wrist Flexors
Wrist Extensors
RECOMMENDED LINKS
Read about the benefits of a proper warm up routine
http://www.fit-2-hit.com - Blog on Athletic Conditioning & Health
http://www.conditioning-coach.com - Tennis & Conditioning Coach