This is the current procedure of becoming a witcher:

Becoming a witcher is a very difficult in-game procedure; only players level 4 may apply to become a witcher. At this point, the procedure is such: After reaching level 4 (which should take about 4-6 months), a player that chooses to become a witcher needs to send me a private message declaring that intention. This will start the player on a journey for some private DM quests that may kill his/her character. There is more than 50% that the character will die on the journey to become a witcher. If successful, the character will be granted witcher status.