In short, you become a proofreader by proofreading. Potential employers or clients are going to be more interested in your track record than they are in a piece of paper from the Proofreading Academy of Greater Manchester (no matter how fine the calligraphy on the certificate).
But how do you get a track record in the first place? Easy, you offer your services for free. If you have any friends or relatives with their own business, offer to proofread the copy on their website or any promotional literature they are producing. Now, create an account on LinkedIn and request recommendations from your ‘clients’. You now have an official business presence complete with testimonials. These testimonials are the true credentials you’ll need to get your foot in the door with a potential employer or client. You don’t need to give away too much for free; just a few hours work should be enough to build up a reasonable portfolio.
And I’m just scratching the surface, here. The No-Nonsense Proofreading Course contains lots more advice on how to wow potential employers.