For simple terms...
You must use parents that carry the piebald gene. If they look normal and carry the piebald gene, they are called "hets" for piebald.
Or you can use a piebald with a "het" to produce more piebalds. But the only way is to use animals that carry that gene.

And "het"s can carry other recessive genes like albino, so make sure you are getting piebald carriers if you decide to use "het" snakes rather than simply buying a piebald. The breeding for getting one is a long process, taking up to 3 years or more(if you buy babys). So in the long run, it may be cheaper and easier to simply buy a piedbald snake, rather than trying to breed your own.
Hope this helps.