If you are trying to get pieds, you may have a long wait. New mutations occur very rarely. If you still want to use only your own stock, look for birds that have a white feather or more. Usually this occurs in the wings. Breed those with a white wingfeather together and see if any of the young have more white feathers than the parents. Inbreed these selected birds together. Keep repeating this process of selection for white feathers. Inbreeding for several generations is not advised, however. Just advertise for pied birds and buy some.
I was director of a large pheasant farm. We hatched over 100,000 chicks per year. We had one albino while I was there (7 years) and I never saw a single pied bird.
Be careful not to bring disease into your flock if you buy from outside.