If you're grating, grate the soap over a large bowl. If you're using a food processor, take your bar of soap and cut it into small pieces. Then, grate/chop it into fine pieces/flakes. (This is another benefit of the Fels Naptha or the 0% superfat bar. They'll tend to grate/chop up much easier.)
A note on fragrance: The Fels Naptha soap has a fragrance added to it...which I don't mind at all. If you've made your own laundry base soap, you can add a fragrance or essential oil of your choice. Many folks will use orange or lavender or some other "clean" smelling scent. Or they'll just leave the scent out completely. The scent doesn't affect the cleaning effectiveness of the soap.