Using a wooden spoon, beat the fudge vigorously until it thickens and begins to lose its gloss. This may take anywhere from 5-15 minutes. While beating by hand is the traditional method, you can use an electric mixer if you have one. Fit the mixer with the paddle attachment and turn it on medium. Beat it just until it starts to thicken and lose its gloss. It is easy to overbeat fudge with a mixer, so watch it carefully and check it after every minute.
Once the fudge is the proper texture, add the nuts and stir quickly just until they are evenly distributed.