I follow the grain lines to layout the centerline of the limbs with a pencil. Then divide the limb into equal parts lengthwise, say 3 - 4" sections(smaller if the stave is super snakey), draw a line across the stave at each division perpendicular to the centerline. Then measure off the centerline at each station to gradually step down the width... i.e. layout the width profile/taper. Match those measurement on the other limb. Then I cut it out on the bandsaw.
In order to taper the bow limbs in width, some gradual violation of the grain must happen. Just do your best to make sure it IS gradual.