How does one figure out the shape of the angled ribs ?

People use CAD to draw a solid wing, then take slices of it. Bob Hunt does his CAD with a slab of foam. I use a spreadsheet.

It's pretty easy to match most any reasonable airfoil (maybe not including flaps) with the modified NACA 4-digit formula given in the Abbott and Costello book. That gives you a sorta-polynomial formula for the airfoil. I did this using the Impact root and tip airfoils. For each point on a slanty rib, you know the local chord, the fraction of that chord that point is behind the LE, and the fraction of span that point is between the root and the tip. Interpolate between the root and tip airfoils to get the airfoil formula for that place on the span. Then calculate the airfoil ordinate for the fraction of the local chord that the point is behind the LE of the local chord. Use that method to calculate the shape of all the ribs. Then there's removing 1/16" or whatever for the sheeting and capstrips. I take pairs of points on the airfoil a hundredth of an inch or so apart, then calculate a perpendicular to the line between them halfway along the line. I go out 1/16" - laser kerf along that perpendicular and locate a point for the final rib. Then take the coordinates of the final rib to a CAD program as a polyline and have the laser cut it.

There's a little extra fussiness. For calculating straight ribs, I use the side of the rib closer to the wing root. For calculating slanty ribs, I take a line from the root side of the rib at the TE to the tip side of the rib at the LE. This gives the right shape after you knock off the protruding edge with a sanding block. You gotta do something similar to get the angled spar slots to come out right.