lost custom CSS after create child theme

Hi, I am using Twenty Fifteen Theme. I went through a long period of time and also your staff helped me with several custom CSS inserted into my theme. My site layout almost finish, but after I created the child theme, I lost all my custom CSS, which did not apply to my child them.

1. How can I get all this custom CSS back to my child theme so my site look like before?
2. After having child theme, from now on, where do I insert my custom CSS?