To make our important job of plate thicknessing much easier, I am painting the bottoms of all the holes with black artists gesso acrylic paint (from artists suppy store).
As a general thing, safety glasses are a great idea.
Here all the hole bottoms have been been painted. Let the paint dry for a few hours.
Next step is to mount the plate you decide to begin with (here the top plate) in a plate holding fixture (dished out inside to accomidate the arching curve on the finished side of the plate) with edge clamp blocks to securely hold the plate.
To make my work easier, I sometimes use my duplicating router system and special guide molds to take out most of the excess (waste) material. This step can also be done with hand gouges. You will discover that the soft spruce wood of the top plate, machines easily while the very tough maple wood of the back is another matter.... use very sharp and high quality tools in either case.
Additionally notice the pencil line around the inside perimeter of the plate drawn when the rib structure was attached earlier. This line is the inside position of the glue liner strips. No wood must be removed beyound this line as this is the glue line to attach to the rib structure.