Cutting ribs by hand with a template is really fast and easy. As Ed said, make a hard template. Plywood works but I prefer to use phenolic board because the knife doesn't nick it and I happen to have plenty. Circuit board or something like that would work too. Of course, plywood works fine but you have to use a little more care.

Once the template is cut and sanded to exact shape, I put a couple of squares of double stick Scotch Tape on the template. Stick it to your shirt a couple of times and put finger prints all over the tape to reduce the tackiness. Now when you place the template on the wood it won't slide around.

Using a new X-acto blade, cut all cross-grain cuts first, ends and notch sides. Then make the outline cuts. Peel the rib off the template and place for the next rib.

Once all the ribs are cut (and make a few spares), test the spar fit in the notches. Then stack all the ribs together with short spars on each side and block sand them. You can even shuffle them a few times to help keep things even. Don't oversand or you'll be cutting new ribs.

You'll be surprised quickly and accurately you can cut a set of ribs by hand with a template.

Dave