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 fill your brain. At first do not over exert force otherwise you will damage you no so touch skull. Gradually over time bump the wall with your head a bit harder. Slowly increase 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 or 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 as a brick. One must first cultivate one’s character before practicing this skill.
NB: I'll not be held responsible for any injuries or suffering if anyone tries this techniques. It requires special guidance froma qualified instructor. I doubt you will find many instructors that know this technique as they are very old techniques and are only usually taught to special students with high potential, dedicated, respect teacher and self-respect, and patience.