At it's most basic, I was thinking it would work this way...
(1) Hold down 'A' key, guy pulls out bow with arrow ready and string pulled back.
(2) While holding down 'A', direct aiming with straight up/down, straight left/right, or diagonal up/down
(3) Let go of 'A' to shoot arrow. Arrow will travel in a physically realistic path determined by angle of shot.
Maybe the player could have a certain amount of time where he could fetch his arrows before they disappear. Also, if there is a way to get a more precise aiming, that'd be ideal. With the above mentioned method, the player would only be able to control the shot by one of 8 aiming directions, and by jumping (shooting mid jump).
Ammo would be quite restricted.