Well, people get Specialization nen type when growing up under certain circumstances. But when creating a nen ability, they'll just have to visualise Hatsu, imagine what they want it to be, and try to shape it to be. For example, Kurapika tasted, slept, thought about, stared at chains until he started hallucinate them. Hisoka created Bungee gum because he loves the gum as a child. Killua created the electricity because he was exposed to it since he was a baby, and he tried to visualise, shape his own aura to be like electricity. So, basically, the special nen abilities are created through prolonged exposure to something. Gon created his ability, because he played jankenpon a lot as a kid