To go from Central (Exchange Square bus terminus) to Stanley, you can take the 6, 6A, or 6X. (I usually just take the 6X since there are fewer stops enroute.)

Sit on the right side going over, unless you have a fear of heights. Don't expect the driver to speak English, but it shouldn't be a problem. Stanley is easily identified by all the tourists with shorts and white socks (and shopping bags).

www.citybus.com.hk has a nice searching feature that shows stops enroute. From your starting point, for example, Central Exchange Square, specify Southern District, Stanley/Ma Hang Area as the destination. What's somewhat humorously called Stanley plaza is the final destination.
Rich