Manifesting is about thinking things into existence. You want that job? you got it. You need some money? you got it.
There are different methods of manifestation. Writing things down repeatedly (I am a model I am a model) , writing a letter to the universe (Dear universe, thank you! I am a successful supermodel) simply saying things in your head while meditating, making vision boards (though I recommend doing that alongside other methods, having a visual aid really helps with manifesting.)
If you doubt yourself, you will not bring anything. Manifesting is about focusing on the end result, not the how. For example, if you're trying to manifest becoming a model, don't ask yourself things like "Am I too short? Am I too fat?" the universe will take care of you and get you there. All you need to worry about, is actually being a model. In order to manifest you must speak in present tense (writing, verbally manifesting) for example: I am a model. I am a successful model. I am on the cover of vogue. etc. You're putting your energy and focus into actually being a model, so that is what you'll receive. If you put all of your energy and focus into worrying about yourself, doubting yourself, and most detrimental to manifestation, doubting the universe... you aren't going to bring what you want into your life.