For citizens of Mainland China, how to visit HK?

You mean you want to enter HK from China mainland?

If so, you need to go to the local Exit-Entry Adminstration Bureau for applying a Two Way Permits and get a register on it. The can only take your Household Register (Hukou) to the bureau. Then you need to fill an application form and then take photo there. Then wait for submit your documents. After you submit the documents, you need to wait to get it.

You visit HK with that permit. BTW, if you get a G register, you can travel to HK on your own. If you get a L register, you need to join a tour to visit HK.