In his The 7 Habits of Highly Effective People, Stephen R. Covey describes the process succintly: "A good affirmation has five basic ingredients: It's personal, it's positive, it's present tense, it's visual, and it's emotional." He then goes on to describe the process of visualizing the affirmation in a state of deep relaxation with eyes closed - hypnosis, in other words. A few minutes is spent each day relaxing and then repeating the affirmation and visualizing it. With repetition a behavior or thought process begins to change and come into line with the affirmation, as this process necessarily makes an impression on the subconscious part of the mind where habitual behaviors and thoughts are stored and triggered. This is one of those built-in tools that everyone has, but relatively few actually take advantage of. Though using affirmations created by others can be effective ("Every day in every way I am getting better and better"), your subconscious knows it isn't you and may not take it in and process it as effectively as you would like.
To create your own affirmation(s), let's break it down.