One each rib, mark two places on the ends to install 1/4" diameter soundboard locator pins (shown in the mid picture). Drill with a 1/4" bit on the mark as deep as the locator pin goes.
Place raw shaped soundboard on top of piano, make sure it covers the entire inner rim and belly. Trace on the bottom of board along the outer rim, mark where belly line is with some dots. Then flip board over, place on a work surface, connect the belly line dots (shown in picture on the right). Clamp board down on work bench.
The raw ribs are made longer than needed for adjustments. Place each rib at place, mark how much to saw off. The marked line is parallel with eighter rim or belly line. Trim conservatively, a little at a time, better leaving too long than too short.