It's fairly complicated. My two boys were both born in China. You'll have to go to the Beijing 驻京办, and they'll ask you to prepare a lot of documents, mostly to prove that you are the baby's real mother. It's best to talk to the HK officer than their local staff. After all the documents are ready, 驻京办 will send them to HK and it takes about 2 months to get the entry permit for the baby. You then take the permit to 公安局 for an exit permit which takes about 1 week.
Now you can take your baby to HK. In HK, you'll have to go to immigration to apply for 居留权 which takes another 2 months (but your baby does not need to be waiting in HK while they process, but at the time of application, the baby must be in HK). Then you can apply for 特区护照. In the meantime, you can apply for 回港证 which is processed right away. With the 回港证, you can apply for 回乡证. But again, it takes 中旅社 1 month to process the application. Once approved, the 回乡证 takes 5 days to get (快证)。
All in all, it's not the easiest process. We're expected our third in Jan, and we've decided to come back to HK for delivery.
Hope this helps, and if you need more info, pls pm me.