This process is surely not ideal. As a small shop, we do the best we can. This is a note of what's done now. Maybe next time we'll improve!
Each rib has two locator holes drilled with 1/4" bit, and locator installed. The locators make marks on soundboard. Make sure the board is pressed down everywhere so all locators transfer marks clearly. Then glue dowels in locator holes on ribs, and trim short.
Drill open the rib to soundboard marks with 1/4" bit. The jig makes sure drill bit goes down straights.
Ribs are pressed on by air pressure. Dry fit before applying glue just to be sure dowels go into locator holes and not too long. We have only limited amount of press, so glue one half at a time.
The next morning, unclamp everything, use jig to feather ribs. The power rotator plane is modified by the shop owner, two sides ride on jig, blade in the middle cuts wood. Steinway B has steps on feather. We shim the feather jig to different height to finish the steps. A cross bar is common on Bs as well. The shop owner routed the cross bar glue surface to let them be on same height.
Use a router and rounded bit to round sides on ribs. Looks much better afterwards.
A power sander would be much more efficient to smooth the feather surface. Soundboard is then put into kiln before further operation. Never forget to clean router bits with special cleaner to sustain their life ban.