Coil soft silk around your forehead about ten times. Now wrap one or two iron pieces around your forehead on the outside (you could use something like chain mail etc). Bump your head against a wall two times only each day. While training you MUST lift your Qi to filly our brain. At first do not over exert force otherwise you will damage you not so tough skull. Gradually over time bump the wall with your head a bit harder. Slowly increasing the force. After more than one year later use seven or eight pieces of silk instead of ten. Then after hundreds of days more practice, decrease the silks to four of five rounds. One more year later you can bump the wall without using silk. It is very painful for the trainee to bump their head straight against a wall but step by step your head should now be as hard a brick. One must first cultivate one’s character before practicing this skill.