Just posting to help anyone else stuck in the misery of trying to get a Police Certificate from China.
As you might expect, the instructions on the CIC website are horrible, inaccurate, and laughably naive.
Here is what I have been able to discover:
1) Technically, you will need to have a separate police certificate for each city you lived in. If you lived in Shanghai and Beijing, each police department will refuse to issue a certificate covering the time you resided in the other city.
2) It is not free.
3) If you're not in China, you will simply have to find an extremely motivated and committed friend - or hire an agent - to work through this process over a period of days or weeks for you.
4) If you need help, you can contact BEJINGIMMIGRATION @ international.gc.ca. Actually, they are not helpful, and they are utterly clueless - but unlike CIC, they will actually respond to you .... which is kind of cool.
5) You will provide a copy of your passport, a copy of the pages of your passport showing the stamps for entering and exiting China, and a letter of authorization IN CHINESE (you can ignore the CIC request letter ... if you manage to even find it on the website ... it's useless in English) stating the name, gender, and ID no. of your agent/friend, along with the dates shown on the stamps in your passport (the dates you were in-country).
And finally, it should be said that these instructions are certainly neither complete nor valid for different times or places in China. No doubt, if you are fluent both in the language and culture where you will ask for the certificate, there are other ways - perhaps even cheap or free. This is just one approach I found work - although at great expense!
Good luck - and please post here if you discover any other effective strategies!