To use autosuggestion you must grasp Coue's "trick".
The training to learn this "trick" as stated by Coue himself in his Self Mastery:
.... This training which perhaps seems to you an impossibility, is, however, the simplest thing in the world ....
Another quote from "how to practice suggestion and auto-suggestion" by Emile Coue when talking about his trick and autosuggestion:
.... It is my part to show you hot it can be done, that it is a very, very simple thing. It is so simple that it is difficult to think that such a simple thing produces such wonderful effects ......
Coue's trick will ultimately show/prove to you:
So the trick is to change the idea in one's mind from "I Can't" to "I Can" without effort.
To change the idea you must not use willpower or effort. Just like any other training you need to train yourself and learn to change this idea without effort.
In another words, everything is controlled by Unconscious, which is the one which runs us and we learn how to run it, through it we learn how to run ourselves. That's the trick!