How are new Chinese characters created?
In ancient time Chinese people were creating new characters. Now they don't. Only new chemical elements create new character.


  What are the rules, what is not allowed, what is allowed?


六書造字原則 is one of the rules.


  Who is allowed to create one? How do you associate a sound with it too? Is it free to create your own?


As far as I know, empirer and his government was allowed to create. 武則天 created 曌 for her own name. There were government officers who controlled the writing system. They might work on characters.