1. Practice complimenting others.
Do this especially for small traits, or particular aspects of an area they might not excel in. For example, someone you don’t find attractive may still have a cute nose, or nice hair. Someone you find annoying may have a friendly attitude, or admirable confidence.
When you learn to do it for others, you can’t help but learn how to do it for yourself.
2. In everyday situations, when you normally would start bashing yourself over an insignificant/small thing, try to think about the situation as if it happened to someone else. Would you hate another person, a classmate, a stranger, whatever, for it? If not, why do you hate yourself for it?
Of course, there ARE things about you that are unpleasant, everybody has flaws. But for that very reason it’s okay to have them. And the harder you work on flaws you can fix, the more respect you deserve.