You can either apply for the China visa from the China embassy in Singapore or from the Visa office of PRC in HK SAR. It will take 4 working days, but there is express service if you are under tight schedule.
If you apply in Hk, you will have to submit the following docoments:
1. Valid passport (at least six months before expiration) with blank visa pages.
2. Completed Chinese visa application form, one colored passport photo taken in the last six months.
3. Under some circumstances, the applicants may be required to provide financial proof, a copy of Chinese hotel booking and return flight ticke.