Mindful breathing is a very simple form of meditation and is a nice one because you can do it anywhere and anytime. I have been taught the following. When you breathe in simply say or think, "Breathing in, I know I am breathing in" and when exhaling simple say or think, "Breathing out, I know I am breathing out". The words can be altered to anything. If you have a pain for example, you can say, "Breathing in I know I have pain", While exhaling, "Breathing out, I know the pain will go away."
The beauty of this practice is that you can be driving a car, walking, even at work and practice.