There was a very good article on this subject published some time back in Photo Techniques magazine. It's available, from the original author, here:

http://www.dokasphotos.com/techniques/ground_glass/

You can obtain the aluminum oxide grinding powder mentioned in the article from Willman Bell. It's available here (scroll down a bit):

http://www.willbell.com/ATMSupplies/ATM_Supplies.htm