All you do is press a button on the controller ,each button corresponds to a -ality except pit fatalities ,they must be entered manually.

I also found that if you press a button to early or too late nothing happens.

Yea, the one button fatalities are very buggy. Sometimes they work and sometimes they don't.

The best timing is right after the Finish Him letters go away. If you do it right when they appear that is too early.

Gah...I've been having problems with that too. Does the difficulty setting have anything to do with whether they work or not? It just seems like I have a much higher success rate on Normal difficulty than I do on Very Easy.