Clamp the plank upright into the workbench vise. Make sure that the saw-guide line is vertical. Start the cut at one corner using the fine, cross-cut teeth of the Ryoba saw. Hold the saw at an angle of 45 degrees and turn it over as soon as the cut is about 1/4" (6mm) deep. This cut goes along the grain and is therefore best done with the larger teeth of the saw. Don't force the saw and don't pull too hard, but hold it gently and hold it steeper (i.e. lower the handle side of the saw) with each stroke. The pulling action of the saw leaves the blade dead straight making it easy to follow the guide line. Continue until you've cut just over the halfway point and halfway down. Flip the plank over and do the same from the other side until both cuts meet at the top.
Remove most of the waste by cutting across the plank at line A. This cut is uncritical, but you must take care not to cut too deep. A piece of veneer or cardboard stuck into the kerf will protect the back of the peghead against being scratched. If you use veneer of a contrasting color, the differently-colored saw dust will tell you when the cut is finished.
Now comes the most difficult cut: the concave transition to the full thickness of the plank. If you have access to a bandsaw with a narrow blade, this is the ideal tool for the job. However, a bow saw - although out of fashion nowadays - will also do the same in a little more time. Clamp the plank to the workbench so that the cut is vertical. All cutting with a bow saw is done on the push stroke, with the teeth facing away from you. Hold the saw with a light grip and let the blade do the work. Follow the guide line as close as possible, but keep it visible all the time. Check frequently if the cut hasn't gone astray at the bottom. If this happens, turn the plank over and correct the cut. With a bit of practice it is possible to make this cut in one pass, although I had to turn the piece over and correct the cut several times. I was lucky because the blade had always ended up in the waste area.
As you can see, the cut turned out not too bad, just slightly off line B. If you manage to cut over the line, draw a new transition line and be more careful when following it. There's some room for corrections; these will, at worst, result in the bridge ending up a bit nearer the peghead.