I have an S2 Chinese visa good for multiple entries of 90 days.

I am in Shanghai and going to Hong Kong to get an additional 90 days in China. I was told all I need to do is go to hong kong stay one day and return and I will get it stamped for an additional 90 days. is this all I need to do? any other paperwork I need to fill out? if so what information do I need to bring with me like amount of money in my account where im staying etc etc