Choose positive affirmations that are not too long.
Repeat them often, when you are not busy with something that requires attention. It could be while traveling in a bus or a train, waiting in line, walking, etc. You may also repeat them in special sessions of 5-10 minutes each, several times a day. Be careful not repeat them while driving or crossing a street.
Be as relaxed as you can while repeating the sentence or sentences you have chosen.
Pay full attention to the words you are repeating.
Believe what you repeating, and feel that your desire has become true, or is in the process of becoming true. This will bring faster results.
Preferably, choose positive words with no negative connotations. If you want to lose weight, don't use words such as, "I am not fat", or "I am losing weight." These are negative statements, bringing into the mind mental images of what you do not want. Repeat instead, "I am getting slim", or "I have my ideal weight". Such words build positive images in your mind.
Use the present tense in your affirmations, not the future tense. Saying, "I will be rich", means that you intend to be rich one day, in the indefinite future, but not now. It is more effective to say, and also to feel, "I am rich now", and the subconscious mind will work at overtime to make this happen now, in the present.
By stating what you want to be true in your life, you mentally and emotionally see and feel it as true, irrespective of your current circumstances, and thereby attract it into your life.