Self-confidence means understanding that you deserve as much love and respect as everyone else. Ideally our family helps us understand this from our very childhood; however, even if we suffer from low self-confidence, it can still be inculcated, although it will take focus and support. My purpose is to give some principles and paradigms of what that effort will look like. If encouragement shines through, and obstacles are made visible, then the answer will be successful and satisfying to my heart.
Let's first distinguish what self-confidence is not; It is not confidence.
By confidence, I mean an attitude generated by someone who has practiced a skill or studied a field of knowledge in such a way that they can do or explain it without fear of embarrassment, and with ambition to show it off. A truly self-confident person may be put into situations which demand them to try out unfamiliar skills or unknown ideas. He or she will naturally feel some insecurity if required to perform or explain those things. Then again, one may be highly trained in a field of knowledge or cultivate staggering technical ability precisely to make up for lack of self-confidence. In either case, no amount of knowledge or skills will substitute for self-value.
Now, one may say, "Well knowledge and skills are important, too!" Indeed they are, yet the attitude of a self-confident person is only minorly affected by possession or lack of them because they know they are "okay", either way. Not so for someone without self-confidence! Their sense of personal value rests on how "good" they think they are at something, or how they believe they're evaluated by others. This locks them into harmful and possibly dangerous emotional dependencies. The two kinds of harm caused by this self-abnegation will be touched on at the end of the answer.
Now, having hopefully distinguished actual self-confidence which is inherent and universal, from confidence in particular skills or knowledge, I'll share from my personal experience how to develop the genuine article, not having absorbed it by childhood.
Growing up largely alone, I felt a lack of care. This was exacerbated by being teased for becoming overweight by fellow school kids. I learned how to win praise by doing art, poetry, scholarship, music and so on, but no matter how, or in which field I excelled, I felt sad or angry because of competing against an inner voice of shame and blame which echoed from those harsh words of my peers. Unfortunately, I didn't realize it at the time, and by my late teens, I turned to drugs, and well... that didn't help matters.
Only after college, when a friend was there for me regardless of my self-hatred, resentment, fear and showing-off, did I finally learn to value someone personally, regardless of their knowledge or skill sets. His self-confidence and his confidence in my self-worth rubbed off. Now I practice it, daily.
Some key paradigms:
"I deserve as much love as the next person; not more, not less."
"Personal worth is an inalienable right of existence, and comes with the responsibility to extend that same respect to others."
A final note: Confusing self-confidence with confidence in a particular knowledge domain, or skill set is harmful in the following ways:
1. To yourself, especially if you blame yourself for feeling inadequate. (Neurosis)
2. To others, especially if you blame other people or situations for feeling inadequate. (Character Disorder)
These manifest when you
a. Can’t master a skill you want,
b. Master a skill but it doesn't satisfy you as you hoped it would,
or (most painfully)
c. When you master a skill, but you lose it.
Legitimate self-confidence, is essential to avoid this needless pain or heal it, and gives the interminable life-strength to experience our shared challenges, progressively together.