"Acting as if" is great advice on building self-confidence. People respond to what you do and how you behave. So if you act and look like you are self-confident, people will treat you as someone who is self-confident.
Self-confident people greet others with a firm handshake, look at them in the eye, and smile. They offer their opinions confidently, and listen attentively to what others have to say.
Self-confident people expect to be accepted. Therefore they are and their self-confidence grows. People who are not self-confident expect to not be accepted. Therefore, they are not and their self-confidence is diminished.
To do this, you have to conquer the fear of rejection. You have to do something e.g initiate a conversation. You have to show that you expect to be accepted.
"Expecting acceptance is putting yourself on par with any other member of the human species. Expecting acceptance is stubbornly and justifiably holding a belief of simple self-acceptance. No one is above you or below you. We are all at the same level."
The key is to create your own positive self-fulfilling prophecy.