Use of discordance requires skill in musicmanship, and an instrument to play. Discordance lowers the skill levels and stats of the target by an amount based on your skill level in discordance.
This skill can be raised above a level of GM up to 120 through the use of power scrolls.
It is always recommend to train any skill from an appropriate NPC before attempting to master it. For bardic training seek out your cities local conservatory of music.
Once you have attained a novice level of skill in discordance you will need to find tougher and tougher creatures to discord as your skills raise.
A common method of training discordance is through the use of pets, if you are not yourself an animal tamer you may find pairing up with one an easy means of access to creatures of varying levels of barding difficulty. Simply work your way up though a list of increasingly difficult pets, by placing them in discord and then clearing discord to avail another attempt. Two means of clearing discord would be by proximity, by either moving away from the target or logging out of the game.