You don't get to decide exactly what the strange mood includes.
The dwarf wants certain things, certain types of things, and you can choose only within those types what he gets. So if he wants, for example, bars of metal, you can choose which type of metal amongst those you have by forbidding all other types. That's about as far as your control goes.
The wiki explains the details better than I ever could: http://df.magmawiki.com/index.php/DF2010:Strange_mood
Since an armorer mood is pretty awesome, you might want to save and keep a backup of this save, such that you can retry if you can't figure out how the strange mood works the first time around.