How to create bootstrap subtheme using sass?
Create a New Sub-theme


Copy bootstrap/starterkits/sass folder into sites/all/themes or a respective sites/*/themes folder.
Rename the folder to a unique machine readable name. This will be your sub-theme's "name". For this example and future examples we'll use subtheme.
Rename subtheme/sass.starterkit to match the folder name and append .info (e.g. subtheme/subtheme.info).
Open subtheme/subtheme.info and change the name, description and any other properties to suite your needs.
Download and extract the latest 3.x.x version of Bootstrap Framework Source Files into the root of your new sub-theme. After it has been extracted, the directory should be renamed (if needed) so it reads subtheme/bootstrap.


Compile Sass


Override default bootstrap variables in subtheme/sass/_default-variables.scss.
Edit subtheme/sass/overrides.scss as your fit to properly integrate with Drupal
Compile subtheme/scss/style.scss to subtheme/css/styles.css.




More Information : Read Drupal-Bootstrap Documentation