General Hints for Hand Training

Take it slowly: don't rush body-hardening or toughening practice!
Punching with even a simple fist can damage the untrained hand.  It takes several weeks for the external flesh to toughen, months for the tendons and bones.  It is very easy to rip the skin off the knuckles when first practicing, and easy to sprain the wrist or finger joints when striking a solid object without the proper alignment.  Whenever an open wound or bruise is created, the hand must be protected and healed before full training can begin again.  Your hands are precious tools for coping with the world -- don't abuse them by trying to quickly transform them into powerful blunt weapons.  True "iron-hand" training requires a specific training sequence, equipment, and proper medicines and liniments.  It should not be attempted without full information and guidance from a skilled practitioner.  Moderate toughening exercise, however, can be safely done with minimal instruction.

Start with light but frequent practice.     
The biggest mistake beginners (and some "advanced" students) make is to try and fit months of conditioning into a couple of weeks or days.  Start with a small but firm target: an old telephone book works well.  Prop it up on a chair or couch, and lightly punch or strike it with the hand-form you are training.  Don't practice striking more than five or ten minutes, or until the knuckles are slightly red.  Immediately afterward, give them and the rest of the hand a gentle massage to encourage more circulation.  Striking lightly, but firmly, will give you immediate feedback in your hand, wrist and arm as you find the proper alignment of the joints.  While going about daily routines and tasks, continue your light training: push open doors, etc. with the knuckles of your closed fist instead of a palm.

Do not train the fingertips on hard objects. 
The tips of the fingers are very sensitive and several acupuncture meridians terminate there.  The Chinese say that hard training on the fingertips without careful preparation and restorative medication can result in poor health and weaker vision.