I started by feeding whole market prawns to my aro. He was around 12" when he started taking food from my hand. Firstly, your arowana has to be comfortable with your presence. Mine swims up to me everyday when I get home.
It takes time and the aro has the be comfortable with you. Not an aro though, but it took me a year to hand feed one of my NTT datnoid and 2 months for my fahaka puffer.