Assuming this is the first time you have been fired or had performance issues, you start by accentuating the positive, perhaps by saying something like.
"I've worked for x companies and have always been a high performer.....", perhaps give examples.
Then you admit that not every job is a perfect fit.
"Sometimes, as in any relationship, things just don't work out. "
Without bashing your company, you make your defense
"In my mind, I did everything I understood I should do. I believe that I behaved ethically and I am comfortable with my behavior; I have no regrets and harbor no ill will against my previous employer. I believe that it was all an unfortunate misunderstanding."
Then you affirm that you have learned from the situation and it will not be repeated;
"In the future, I will make sure that all of my transactions are completely open, so there will be no room for miscommunication. "
Such an approach, I believe, goes a long way to diffuse the situation.
Under no circumstances let yourself be roped into bashing your previous company or managers. There is no scenario in which that helps your case.
How you spin a sacking for cause is another question.
– Dec 22 '16 at 21:22