Fill Flash and Aperture

[sanyasi]

I have read a number of articles about fill flash and setting your flash exposure one or two stops below the exposure that you are using in the camera. I have read that the Nikon system allows you to do this by dialing in an ISO and aperture setting into the flash--you also need to factor distance in, which apparently the Nikon system does. I don't see how you would do this with a Canon Speedlite 580EX II. Am I confusing concepts?

Any assistance would be appreciated.

Jack Siegel