Just create the sub-theme folder manually. The folder name you'd use for it will be the machine name of your subtheme.
For more details, refer to:
- "Creating a sub-theme" (= Community documentation that applies to any sub-theme).
- "Sub-theming" (= documentation about sub-theming specific to Drupal's Bootstrap theme).
If you prefer to look at a great (I think) example of a (contributed) Bootstrap sub-theme, then have a look at the Bootstrap Business theme.
Note: the question about "http://drupal.stackexchange.com/questions/141538" contains a lot of details to get started with such sub-theme ...