just be quick, brutal and to the point, say what you need to say, neutral place,maybe somewhere noisy and with lots of witnesses incase it gets ugly.
If its because you are sick to death of 4am phone calls and the fact that she has no sense of humour, say that but dont do it in an insulting way.... and dont ever ever be appologetic, you are doing it for a reason appologising shows you are not behind it 100%
it should take 10mins max for you to say your peice and give her plenty of opertunity to respond...then get the hell out of there and cut off all contact.