This tutorial will show you how to use each feature of the Cartridge Case Identification Virtual Comparison Microscop (CCID VCM) found at http://www.firearmsid.com/ (of which I have no affiliation)
You will be comparing the cartridge casings of unknowns to known standards in order to try and find a match. As you progress to the next CCID tests the difficulty increases. Some of the unknowns may not have a match among the standards, and some unknowns may match to the same standard! Good luck!