Please browse this site for everything you want to know about China Visas from Hong Kong - from the China Foreign Ministry in Hong Kong website
http://www.fmcoprc.gov.hk/eng/zgqz/blsjfy/
Also, I suggest strongly that you do NOT use any travel agency etc including China Travel Service - for all they do is go take your passport and apply the same way that you would do yourself; the only difference being that you do NOT get an official receipt for your passport (and I've had a friend get into a real pickle with the travel agent "misplacing" his passport) PLUS it will cost you more.
Only if you insist on not lining up yourself, (which CAN be quite lengthy at times, depending on the time and any holidays/festivals etc coming up) then it's not a complicated process to apply yourself.
SEAJ