Whenever possible rib stock should be cut off the wood used for the back of the instrument. Rib stock long enough to make a joint-less lower bout is very desirable, and TMB likes to do a one piece lower bout whenever he has the material to do so.
Ribs are ripped off the back at +2mm. They are then ripped to a width of 32mm. It helps to plane one edge flat (jointer plane set upside down in a bench vise). This edge is then registered against the bandsaw fence, when ripping to width. Alternatively, the excess wood on the back can be ripped off in one large chunk, flattened with a plane on one side, squared 90 degrees to form a "bottom" edge, then ripped to 33mm. After that the ribs can be ripped off at 1.9 - 2mm. This method has some advantages over ripping for thickness right off the back, namely, it is easier to control the thickness being cut over a span of 33mm than a span of 70 - 100mm.
A pine plank is placed on top of the bench and the far end held down with a wooden cam clamp. The rib stock is placed on the near end and held in place with a large F-clamp. One side of the stock is flattened and smoothed. At this point it would be good to mention flame direction. Generally the flame direction of the upper and lower bouts are the same, and the c-bout runs in an opposite direction. Once you've identified which direction you want the flames to go, proceed with flattening and smoothing the outside of the rib. Large variations in the surface are reduced with a toothed blade in a block plane. Switch ends and plane the end of the rib that was under the clamp. Once the surface is brought down to a common level, use a sharp scraper to remove the marks left from the toothed blade. ( With some wood of little flame it is possible to use a standard plane blade without tearing out the wood.) The scraper should be used in long even strokes, being careful not to tip the scraper to one side or the other, which would thin one edge too much. Initially, when removing a lot of wood with the scraper the blade should be angled so that the direction of travel is roughly 90 degrees to the flame of the maple. In this way, the scraper is supported across several flames at once, minimizing the tendency of the scraper to dig-in between the flames. Once the marks from the toothed blade are scraped out and the surface of the rib is smooth, the direction of the scraper is changed to parallel the flames. Light passes are made in this way to create subtle waves in the surface of the rib which will help to develop visual contrast during the varnishing process.
Mark the smoothed outside of each rib, and proceed to thicknessing the inside of the ribs. At this point the rib stock will likely still be thicker than 1.5mm. Bring the thickness down to 1.5mm with the block plane. Again, switch ends and thickness the stock that was under the clamp. Proceed with a scraper as described above. Carefully watch progress and check frequently with a thickness caliper. The finished thickness should be a consistent 1.2mm both across the width and down the entire length of the rib. Variations from 1.1mm to 1.3mm are acceptable, although the more consistent the thickness, the easier the wood is to work with when bending the ribs.(Anything less than 1.1mm is unacceptable. Anything greater than 1.3mm needs more work. ) Once a thickness of 1.2mm has been established (verified with the caliper) this area on the rib can be used as a visual reference when viewed from the side. In this way it becomes unnecessary to constantly remove the rib from the clamp to check thickness. The inside of the rib doesn't need to be finished to the same degree as the outside, and often due to thickness concerns is left with visible tool marks.