Use of peacemaking requires skill in musicmanship, and an instrument to play. Peacemaking can be used in two ways, by targeting a creature or beast to remove them from combat for a short time. Peacemaking can also be used by targeting yourself, this will attempt to remove nearby targets (players and mobs) from war mode, this does not prevent targets from immediately re-entering combat.
Although able to be trained on mobs of increasing difficulty, one of the easiest ways to train this skill in practice is in a location with a large number of players (think: Britain bank) around and targeting yourself. A simple macro to use an available instrument then use peacemaking and targeting yourself while in range of multiple targets will sufficiently train this skill to GM.