Fill Flash

I have a 20D and a 580EX, and trying to figure out how to use as Fill Flash. Can't find anything in either manual about it

a little complicated, but here's everything you need to know on Canon flashes: http://photonotes.org/​articles/eos-flash/

And here's the fill flash part:
http://photonotes.org …ash/index2.html​#fillflash

Super (over) simplified: Fill flash, put Canon camera in Av or Tv mode,turn on flash, aim and shoot!
The comlications: A) doing the above and not being aware that in low light setting the camera in Av can and will select a very sllloooowww shutter speed to pick up ambient light to the appropriate level (while exposing main subject via ETTL flash) will result in motion blurred shots because you are assuming the camera has a decent handholdable shutter speed. B) what the camera truly does is determine in part by the Custom Fn for selection of flash synch speed C) the camera behaves differently above EV10 light level vs. below EV10 light level

Here is everything that you need to know about fill flash:
http://www.popphoto.co​m …download/821200​311318.pdf
or (same article)
http://www.popphoto.co​m …ction_id=4&arti​cle_id=706

This will give you a great start but obviously, you always have to consider these additional things also (max & min shutter speed, over exposing sky/circular polarizer/lens flare, position of shadows across a person's face).