A stationary disc sander with 80 or 120 grit paper is the ideal tool for this job. Measure from the top of the existing crown of the bridge (not the bottom of the existing string groove). You should end up with four points (e, a, d, g). Now imitate the existing bridge crown curvature and connect these four dots. Also, at each dot, draw a straight line down from the crown of the bridge towards the feet. You will end up with four parallel and vertical lines. These lines will act as a reference for your string spacing.
Disc sand to this new bridge curvature line. GO SLOW. You can always remove wood, but it is damn hard to add wood.

You will also need a small triangle file and small needle file to start/widen the string grooves to the required diameter. The crown of the bridge shouldn't be a square edge; more of a gentle curve. You will need to put a chamfer on the front and back of the new bridge crown. Use a bastard file and some sandpaper to establish this gentle profile.
Make sense?