It's not magical. Affirmations work on your brain. In your brain there is a filter. This filter keeps you from being overwhelmed by your senses. Have you ever sat in a quiet room, and then the power went out and you noticed how it wasn't actually so quiet before? That is because the filter filters out those unimportant noises most of the time, they become much more important though when they are gone.
Imagine next you're in a crowded place. You hear people talking but you can't make out all the different conversations. However if anyone were to call out your name, that would register immediately because your brain is programmed to filter in your own name. Your brain knows that someone calling your name is important. Lastly did you ever notice how many people drive the same car you drive? You probably never noticed that before you bought your current vehicle, but you noticed it immediately right after you started to drive it.
Affirmations work by programming the brain.
Affirmations should be stated in the positive, you can't say "I am not eating chocolate" Just like if I told you to meditate for a moment on any animal in the world except for a polar bear, the first image you'd hold would be that of a polar bear. Instead affirm " I make healthy eating choices throughout the day"
Affirmations should be positive, and be stated in the present, the brain hates lying and being wrong, so if you say "I feel amazing at my target weight of 140 pounds" your brain knows this not to be true, and will work to make it true.
Lastly affirmations should have some feeling. Affirmations should convey to you a sense of peace and happiness when you recite them. You can write them on paper, or create compelling images to look at, you can read them out loud and listen to them often, whatever works for you. Choose just a few to get started. Here are a few of mine.