I could never make sense of Sheldon's trick but if you have one chain whip it's really easy. Shift the bike into the small-large combo (lowest gear.) That will hold the cassette from spinning backwards. Then use your chain whip on the small cog to remove it.

Regarding fixed cog removal (and installation) the Rotafix method is by far my favorite and doesn't require any tools besides the bike itself and a length of chain you don't care about. Google "Rotafix" and there should be some good instructions with pictures. It's really easy and will get the chain tighter than any other method I'm aware of.