Help needed converting WordPress theme into a child theme

Hi! So, a few years ago, a friend paid someone to create a custom WordPress theme for her WordPress based website. For whatever reason, this person modified the base TwentyTen theme *directly* instead of creating a child theme. *sigh* So, I can’t upgrade the TwentyTwelve theme without destroying the site.

So, what’s the best, fastest, and/or easiest way to convert the modified TwentyTwelve WordPress theme into a child theme, so I can safely keep the stock TwentyTwelve theme up to date?

Thanks in advance for your time and assistance!