I’m not exactly a self-help expert, but I’d assume it’s through putting yourself out there to improve your social skills as much as possible.

If you’re genuinely socially stunted/have a legitimate social anxiety disorder, it probably means getting therapy or seeing a psychiatrist.

If you’re just kind of socially awkward, it probably means joining public speaking organizations like Toastmasters. If you’re still in a high school or college, take an improv class or an acting class in your schools theatre department. Also, at this point, genuinely try to make friends and go out with people as much as possible.

If you’re average to already somewhat charismatic, the sky is the limit... do anything I mentioned before, just get way more deeply involved. Apply yourself in social situations as much as possible. Get invited to parties and go to networking conferences and events, and actively try to take up leadership positions.

TL;DR - If genuinely mentally ill or socially stunted - see a therapist or psychiatrist. Otherwise, Toastmasters, improv, theatre/acting, and just in general applying yourself.