Drupal 8 - How to create a sub-theme
How to create a Bootstrap LESS sub-theme in 6 steps:
1 Download Bootstrap theme from https://www.drupal.org/project/bootstrap
2 Copy LESS folder from the themes/bootstrap/starterkit to themes/bootstrap/
3 Rename LESS to mytheme and all THEMENAME occurrences to mytheme.
4 Download bootstrap source code form http://getbootstrap.com/getting-started/#download and extract the files under themes/mytheme/bootstrap directory. If bootstrap directory is missing you should create it.
5 Go to admin/appearance and enable the theme.
6 You also need a less processor for bootstrap files (like: LiveReload). Please follow this http://feedback.livereload.com/knowledgebase/articles/67441-how-do-i-start-using-livereload to install and configure LiveReload.