In simple words, autosuggestion is a self-suggestion through which all the dominating thoughts enter your subconscious mind and affect it and that in turn affect your attitude and behavior in life. The thought which gets repeated or occupy maximum time in your conscious mind automatically become dominating thoughts and enter your subconscious mind. In order to make positive changes into your life, you have to be aware about what kind of thoughts your conscious mind perceives through sense organs and focuses on the positive one so that it becomes dominating and enter your subconscious mind.
You cannot control the actions of your subconscious mind directly with ease but you can modify its effect by controlling your conscious mind. Once you control your conscious mind, you can control your subconscious mind indirectly. Your conscious mind is a platform where all actions begin. Through conscious autosuggestion, you can plant the seed of positive thoughts to make your life much more fulfilling by realizing your desire.
Further, whatever you intend to achieve in your life, make proper use of autosuggestion to translate your desire into reality. For example, if you intend to be fluent in any particular language and working upon it, phrases your desire into the progressive present tense like." I am becoming the confident, in particular, language, day by day." Add your emotions while chatting your desire twice in a day for at least one months and visualize yourself as a confident speaker in that language. Once you start following this step, your subconscious mind will attract all the necessary ingredient to make you confident in that language.
Finally, always phrase your desire into positive sentence. However, mere chanting of phrases will not help you to realize your desired goal unless combined with your emotions and visual of yourself in already possession of that desired goal. Keep in mind, autosuggestion is a great tool to program your subconscious mind to translate your desire into reality.