Reward the child. Fortunately, God is merciful, and children are easier to train than spouses. No child in the history of the world ever improved their talking behavior by being told they are indulging in 'back talk'. The phrase is too vague, general, unfair, and basically it is indulging in name calling. Sometimes for some people it is harder to be simple, relaxed and funny than to be serious.
Simple methods work when they are used by a relaxed person who enjoys a sense of humor.
You have to think out what you want in objective detail. Be willing to give a bunch of rewards for no reason at all. Watch for the smallest most nearly correct action and reward it immediately. Give rewards immediately for approximations of the next step. Repeat and go back through the process adding little step by little step.