How to Create Sub-Theme in Drupal 7 for Bootstrap Project
How to Create Bootstrap SubTheme in Drupal 7
A full instruction to create sub-theme in Drupal 7
Official Guide to create subtheme
https://www.drupal.org/docs/7/theming/creating-a-sub-theme
Download Bootstrap Theme for Drupal 7
https://www.drupal.org/project/bootstrap