After frying the noodles in this way for 1-2 minutes, add the bean sprouts plus about 1/4 tsp. freshly ground black pepper (sprinkle the black pepper over). Continue "tossing" for 1 more minute, or until noodles are done.
When done, the noodles will taste soft but still chewy ("al dente") and a little sticky. At this point, you can add more fish sauce if you don't find the noodles salty enough (I usually add about 1 more Tbsp.).
Note: Do not add any more liquid (other than fish or chili sauce) as you finish stir-frying, or your noodles will become "mushy". If your wok/frying pan is too dry, push noodles aside and add 1-2 Tbsp. oil, then continue cooking the noodles until done.