Steinway & Sons, Model S, using sitka spruce.
Select wood of similar color, cut to length, about 2" longer than soundboard pattern on two ends of each piece of wood. Plane wood 1mm thicker than final thickness. The final thickness is based on the original board or requirements from piano rebuilders who ordered the board. Before machining, mark both sides of each board with pencil or chock. The machine will take off the marks. No marks left means no low spots missed.
Put the wood pieces in order, check warping. The warps shall go down, same with soundboard crown. If the wood is warping upwards (left), flip it over (right).
Check if a nod is at a visible location (left). If possible, switch to where it'll be under the plate (right). Check that the switch won't affect the final shape as a whole piece.
On each piece, rout tongue (left) on one side and groove (mid) on the other side. All tongues are facing the same direction, so are grooves. The work scene will be very messy (right). It's ok, as we kept telling ourselves.
Put all pieces together, check untight joints. Oops, there is a gap (left). Re-rout the non-flat edge, check again and again. Last fitting check, see if the whole piece is still bigger than pattern. If smaller, add wood. Trace the shape of the pattern (mid), mark the end of each piece, that will be where we stop applying glue (right).
Locate clamps of different length where they fit. Mark location of clamps (left). Measure length of board where the clamps are (mid), select cauls accordingly (right). The cauls will be on top and bottom of soundboard, in between the clamps.
Ready to glue. Place tarp under clamps to catch glue (left). Dry fit before gluing up. Apply glue between the marks we just did where the neighboring pieces end (right).
All clamped up. Check gaps again, make sure the clamps are tight (left). Clean glue squeeze outs so it will be easier to sand after glue dries. We'll wait for a couple days before unclamping this big boy/girl.