Sub-themes are just like any other theme, with one difference: They inherit the parent theme's resources. There are no limits on the chaining capabilities connecting sub-themes to their parents. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential.
Then you need to add the following line to my_subtheme.info file to declare its parent or "base theme.": change "theme_name" to the internal name of the parent theme (that is, the name of the parent theme's .info file, usually all lower case).
base theme = theme_name